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, 39 insertions, 6 deletions

diff --git a/src/zenserver/sessions/sessions.h b/src/zenserver/sessions/sessions.h
index db9704430..8f07bfc31 100644
--- a/src/zenserver/sessions/sessions.h
+++ b/src/zenserver/sessions/sessions.h
@@ -11,6 +11,7 @@ ZEN_THIRD_PARTY_INCLUDES_START
 #include <tsl/robin_map.h>
 ZEN_THIRD_PARTY_INCLUDES_END
 
+#include <deque>
 #include <optional>
 #include <string>
 #include <vector>
@@ -33,10 +34,20 @@ public:
 	{
 		Oid			Id;
 		std::string AppName;
+		std::string Mode;
 		Oid			JobId;
 		CbObject	Metadata;
 		DateTime	CreatedAt;
 		DateTime	UpdatedAt;
+		DateTime	EndedAt{0};
+	};
+
+	struct LogEntry
+	{
+		DateTime	Timestamp;
+		std::string Level;
+		std::string Message;
+		CbObject	Data;
 	};
 
 	class Session : public TRefCounted<Session>
@@ -51,23 +62,44 @@ public:
 		const SessionInfo& Info() const { return m_Info; }
 		void			   UpdateMetadata(CbObjectView Metadata)
 		{
-			// Should this be additive rather than replacing the whole thing? We'll see.
 			m_Info.Metadata	 = CbObject::Clone(Metadata);
 			m_Info.UpdatedAt = DateTime::Now();
 		}
 
+		void SetEndedAt(DateTime When) { m_Info.EndedAt = When; }
+
+		void				  AppendLog(LogEntry Entry);
+		std::vector<LogEntry> GetLogEntries(uint32_t Limit = 0, uint32_t Offset = 0) const;
+		uint64_t			  GetLogCount() const;
+
+		/// Returns entries appended after the given cursor and the new cursor value.
+		/// A cursor of 0 returns all entries currently in the deque.
+		struct CursorResult
+		{
+			std::vector<LogEntry> Entries;
+			uint64_t			  Cursor;  // new cursor for next poll
+			uint64_t			  Count;   // current deque size
+		};
+		CursorResult GetLogEntriesAfter(uint64_t AfterCursor) const;
+
 	private:
-		SessionInfo		m_Info;
-		Ref<SessionLog> m_Log;
+		SessionInfo			 m_Info;
+		Ref<SessionLog>		 m_Log;
+		mutable RwLock		 m_LogLock;
+		std::deque<LogEntry> m_LogEntries;
+		uint64_t			 m_TotalAppended = 0;  // monotonically increasing counter
+
+		static constexpr uint32_t MaxLogEntries = 10000;
 	};
 
 	SessionsService();
 	~SessionsService();
 
-	bool					  RegisterSession(const Oid& SessionId, std::string AppName, const Oid& JobId, CbObjectView Metadata);
-	bool					  UpdateSession(const Oid& SessionId, CbObjectView Metadata);
-	Ref<Session>			  GetSession(const Oid& SessionId) const;
+	bool		 RegisterSession(const Oid& SessionId, std::string AppName, std::string Mode, const Oid& JobId, CbObjectView Metadata);
+	bool		 UpdateSession(const Oid& SessionId, CbObjectView Metadata);
+	Ref<Session> GetSession(const Oid& SessionId) const;
 	std::vector<Ref<Session>> GetSessions() const;
+	std::vector<Ref<Session>> GetEndedSessions() const;
 	bool					  RemoveSession(const Oid& SessionId);
 	uint64_t				  GetSessionCount() const;
 
@@ -77,6 +109,7 @@ private:
 	LoggerRef									   m_Log;
 	mutable RwLock								   m_Lock;
 	tsl::robin_map<Oid, Ref<Session>, Oid::Hasher> m_Sessions;
+	std::vector<Ref<Session>>					   m_EndedSessions;
 	std::unique_ptr<SessionLogStore>			   m_SessionLogs;
 };