Dashboard refresh (logs, storage, network, object store, docs) (#835)

## Summary

This PR adds a session management service, several new dashboard pages, and a number of infrastructure improvements.

### Sessions Service
- `SessionsServiceClient` in `zenutil` announces sessions to a remote zenserver with a 15s heartbeat (POST/PUT/DELETE lifecycle)
- Storage server registers itself with its own local sessions service on startup
- Session mode attribute coupled to server mode (Compute, Proxy, Hub, etc.)
- Ended sessions tracked with `ended_at` timestamp; status filtering (Active/Ended/All)
- `--sessions-url` config option for remote session announcement
- In-process log sink (`InProcSessionLogSink`) forwards server log output to the server's own session, visible in the dashboard

### Session Log Viewer
- POST/GET endpoints for session logs (`/sessions/{id}/log`) supporting raw text and structured JSON/CbObject with batch `entries` array
- In-memory log storage per session (capped at 10k entries) with cursor-based pagination for efficient incremental fetching
- Log panel in the sessions dashboard with incremental DOM updates, auto-scroll (Follow toggle), newest-first toggle, text filter, and log-level coloring
- Auto-selects the server's own session on page load

### TCP Log Streaming
- `LogStreamListener` and `TcpLogStreamSink` for log delivery over TCP
- Sequence numbers on each message with drop detection and synthetic "dropped" notice on gaps
- Gathered buffer writes to reduce syscall overhead when flushing batches
- Tests covering basic delivery, multi-line splitting, drop detection, and sequencing

### New Dashboard Pages
- **Sessions**: master-detail layout with selectable rows, metadata panel, live WebSocket updates, paging, abbreviated date formatting, and "this" pill for the local session
- **Object Store**: summary stats tiles and bucket table with click-to-expand inline object listing (`GET /obj/`)
- **Storage**: per-volume disk usage breakdown (`GET /admin/storage`), Garbage Collection status section (next-run countdown, last-run stats), and GC History table with paginated rows and expandable detail panels
- **Network**: overview tiles, per-service request table, proxy connections, and live WebSocket updates; distinct client IPs and session counts via HyperLogLog

### Documentation Page
- In-dashboard Docs page with sidebar navigation, markdown rendering (via `marked`), Mermaid diagram support (theme-aware), collapsible sections, text filtering with highlighting, and cross-document linking
- New user-facing docs: `overview.md` (with architecture and per-mode diagrams), `sessions.md`, `cache.md`, `projects.md`; updated `compute.md`
- Dev docs moved to `docs/dev/`

### Infrastructure & Bug Fixes
- **Deflate compression** for the embedded frontend zip (~3.4MB → ~950KB); zlib inflate support added to `ZipFs` with cached decompressed buffers
- **Local IP addresses**: `GetLocalIpAddresses()` (Windows via `GetAdaptersAddresses`, Linux/Mac via `getifaddrs`); surfaced in `/status/status`, `/health/info`, and the dashboard banner
- **Dashboard nav**: unified into `zen-nav` web component with `MutationObserver` for dynamically added links, CSS `::part()` to merge banner/nav border radii, and prefix-based active link detection
- Stats broadcast refactored from manual JSON string concatenation to `CbObjectWriter`; `CbObject`-to-JS conversion improved for `TimeSpan`, `DateTime`, and large integers
- Stats WebSocket boilerplate consolidated into `ZenPage.connect_stats_ws()`

1 files changed, 119 insertions, 22 deletions

diff --git a/src/zenserver/sessions/sessions.cpp b/src/zenserver/sessions/sessions.cpp
index d919db6e9..1212ba5d8 100644
--- a/src/zenserver/sessions/sessions.cpp
+++ b/src/zenserver/sessions/sessions.cpp
@@ -46,6 +46,78 @@ SessionsService::Session::Session(const SessionInfo& Info) : m_Info(Info)
 }
 SessionsService::Session::~Session() = default;
 
+void
+SessionsService::Session::AppendLog(LogEntry Entry)
+{
+	RwLock::ExclusiveLockScope Lock(m_LogLock);
+	m_LogEntries.push_back(std::move(Entry));
+	++m_TotalAppended;
+	while (m_LogEntries.size() > MaxLogEntries)
+	{
+		m_LogEntries.pop_front();
+	}
+}
+
+std::vector<SessionsService::LogEntry>
+SessionsService::Session::GetLogEntries(uint32_t Limit, uint32_t Offset) const
+{
+	RwLock::SharedLockScope Lock(m_LogLock);
+
+	const uint32_t Total = uint32_t(m_LogEntries.size());
+	if (Offset >= Total)
+	{
+		return {};
+	}
+
+	const uint32_t Available = Total - Offset;
+	const uint32_t Count	 = (Limit > 0) ? std::min(Limit, Available) : Available;
+
+	std::vector<LogEntry> Result;
+	Result.reserve(Count);
+	for (uint32_t i = Offset; i < Offset + Count; i++)
+	{
+		Result.push_back(m_LogEntries[i]);
+	}
+	return Result;
+}
+
+uint64_t
+SessionsService::Session::GetLogCount() const
+{
+	RwLock::SharedLockScope Lock(m_LogLock);
+	return m_LogEntries.size();
+}
+
+SessionsService::Session::CursorResult
+SessionsService::Session::GetLogEntriesAfter(uint64_t AfterCursor) const
+{
+	RwLock::SharedLockScope Lock(m_LogLock);
+
+	const uint64_t DequeSize = m_LogEntries.size();
+
+	// Cursor 0 means "give me everything currently in the deque".
+	// Otherwise, compute how many new entries were appended since the cursor.
+	uint64_t NewCount = (AfterCursor == 0) ? DequeSize : (m_TotalAppended > AfterCursor ? m_TotalAppended - AfterCursor : 0);
+
+	// Clamp to what's actually available in the deque (entries may have been evicted).
+	NewCount = std::min(NewCount, DequeSize);
+
+	std::vector<LogEntry> Result;
+	Result.reserve(NewCount);
+
+	const uint64_t StartIndex = DequeSize - NewCount;
+	for (uint64_t i = StartIndex; i < DequeSize; i++)
+	{
+		Result.push_back(m_LogEntries[i]);
+	}
+
+	return CursorResult{
+		.Entries = std::move(Result),
+		.Cursor	 = m_TotalAppended,
+		.Count	 = DequeSize,
+	};
+}
+
 //////////////////////////////////////////////////////////////////////////
 
 SessionsService::SessionsService() : m_Log(logging::Get("sessions"))
@@ -55,25 +127,31 @@ SessionsService::SessionsService() : m_Log(logging::Get("sessions"))
 SessionsService::~SessionsService() = default;
 
 bool
-SessionsService::RegisterSession(const Oid& SessionId, std::string AppName, const Oid& JobId, CbObjectView Metadata)
+SessionsService::RegisterSession(const Oid& SessionId, std::string AppName, std::string Mode, const Oid& JobId, CbObjectView Metadata)
 {
-	RwLock::ExclusiveLockScope Lock(m_Lock);
-
-	if (m_Sessions.contains(SessionId))
+	// Log outside the lock scope — InProcSessionLogSink calls back into
+	// GetSession() which acquires m_Lock shared, so logging while holding
+	// m_Lock exclusively would deadlock.
 	{
-		return false;
+		RwLock::ExclusiveLockScope Lock(m_Lock);
+
+		if (m_Sessions.contains(SessionId))
+		{
+			return false;
+		}
+
+		const DateTime Now = DateTime::Now();
+		m_Sessions.emplace(SessionId,
+						   Ref(new Session(SessionInfo{.Id		  = SessionId,
+													   .AppName	  = AppName,
+													   .Mode	  = Mode,
+													   .JobId	  = JobId,
+													   .Metadata  = CbObject::Clone(Metadata),
+													   .CreatedAt = Now,
+													   .UpdatedAt = Now})));
 	}
 
-	ZEN_INFO("Session {} registered (AppName: {}, JobId: {})", SessionId, AppName, JobId);
-
-	const DateTime Now = DateTime::Now();
-	m_Sessions.emplace(SessionId,
-					   Ref(new Session(SessionInfo{.Id		  = SessionId,
-												   .AppName	  = std::move(AppName),
-												   .JobId	  = JobId,
-												   .Metadata  = CbObject::Clone(Metadata),
-												   .CreatedAt = Now,
-												   .UpdatedAt = Now})));
+	ZEN_INFO("Session {} registered (AppName: {}, Mode: {}, JobId: {})", SessionId, AppName, Mode, JobId);
 	return true;
 }
 
@@ -126,20 +204,39 @@ SessionsService::GetSessions() const
 bool
 SessionsService::RemoveSession(const Oid& SessionId)
 {
-	RwLock::ExclusiveLockScope Lock(m_Lock);
+	std::string RemovedAppName;
+	Oid			RemovedJobId;
 
-	auto It = m_Sessions.find(SessionId);
-	if (It == m_Sessions.end())
 	{
-		return false;
-	}
+		RwLock::ExclusiveLockScope Lock(m_Lock);
+
+		auto It = m_Sessions.find(SessionId);
+		if (It == m_Sessions.end())
+		{
+			return false;
+		}
+
+		RemovedAppName = It.value()->Info().AppName;
+		RemovedJobId   = It.value()->Info().JobId;
+
+		Ref<Session> Ended = It.value();
+		Ended->SetEndedAt(DateTime::Now());
+		m_EndedSessions.push_back(std::move(Ended));
 
-	ZEN_INFO("Session {} removed (AppName: {}, JobId: {})", SessionId, It.value()->Info().AppName, It.value()->Info().JobId);
+		m_Sessions.erase(It);
+	}
 
-	m_Sessions.erase(It);
+	ZEN_INFO("Session {} removed (AppName: {}, JobId: {})", SessionId, RemovedAppName, RemovedJobId);
 	return true;
 }
 
+std::vector<Ref<SessionsService::Session>>
+SessionsService::GetEndedSessions() const
+{
+	RwLock::SharedLockScope Lock(m_Lock);
+	return m_EndedSessions;
+}
+
 uint64_t
 SessionsService::GetSessionCount() const
 {