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, 0 deletions

diff --git a/src/zenserver/sessions/inprocsessionlogsink.cpp b/src/zenserver/sessions/inprocsessionlogsink.cpp
new file mode 100644
index 000000000..9982859b6
--- /dev/null
+++ b/src/zenserver/sessions/inprocsessionlogsink.cpp
@@ -0,0 +1,39 @@
+// Copyright Epic Games, Inc. All Rights Reserved.
+
+#include "inprocsessionlogsink.h"
+
+#include <zencore/logbase.h>
+
+namespace zen {
+
+/// Bias in seconds from DateTime epoch (year 1) to Unix epoch (1970).
+static constexpr uint64_t UnixEpochBiasSeconds = uint64_t(double(1970 - 1) * 365.2425) * 86400;
+
+static DateTime
+TimePointToDateTime(logging::LogClock::time_point Time)
+{
+	auto	 Duration = Time.time_since_epoch();
+	auto	 Seconds  = std::chrono::duration_cast<std::chrono::seconds>(Duration);
+	uint64_t Ticks	  = (UnixEpochBiasSeconds + static_cast<uint64_t>(Seconds.count())) * TimeSpan::TicksPerSecond;
+	return DateTime{Ticks};
+}
+
+void
+InProcSessionLogSink::Log(const logging::LogMessage& Msg)
+{
+	Ref<SessionsService::Session> Session = m_Service.GetSession(m_SessionId);
+	if (!Session)
+	{
+		return;
+	}
+
+	SessionsService::LogEntry Entry{
+		.Timestamp = TimePointToDateTime(Msg.GetTime()),
+		.Level	   = std::string(logging::ToStringView(Msg.GetLevel())),
+		.Message   = std::string(Msg.GetPayload()),
+	};
+
+	Session->AppendLog(std::move(Entry));
+}
+
+}  // namespace zen