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, 31 insertions, 2 deletions

diff --git a/src/zenserver/sessions/httpsessions.h b/src/zenserver/sessions/httpsessions.h
index e07f3b59b..a5783a46b 100644
--- a/src/zenserver/sessions/httpsessions.h
+++ b/src/zenserver/sessions/httpsessions.h
@@ -5,16 +5,25 @@
 #include <zenhttp/httpserver.h>
 #include <zenhttp/httpstats.h>
 #include <zenhttp/httpstatus.h>
+#include <zenhttp/websocket.h>
 #include <zentelemetry/stats.h>
 
+ZEN_THIRD_PARTY_INCLUDES_START
+#include <asio/io_context.hpp>
+#include <asio/steady_timer.hpp>
+ZEN_THIRD_PARTY_INCLUDES_END
+
 namespace zen {
 
 class SessionsService;
 
-class HttpSessionsService final : public HttpService, public IHttpStatusProvider, public IHttpStatsProvider
+class HttpSessionsService final : public HttpService, public IHttpStatusProvider, public IHttpStatsProvider, public IWebSocketHandler
 {
 public:
-	HttpSessionsService(HttpStatusService& StatusService, HttpStatsService& StatsService, SessionsService& Sessions);
+	HttpSessionsService(HttpStatusService& StatusService,
+						HttpStatsService&  StatsService,
+						SessionsService&   Sessions,
+						asio::io_context&  IoContext);
 	virtual ~HttpSessionsService();
 
 	virtual const char* BaseUri() const override;
@@ -24,6 +33,13 @@ public:
 	virtual void	 HandleStatsRequest(HttpServerRequest& Request) override;
 	virtual void	 HandleStatusRequest(HttpServerRequest& Request) override;
 
+	void SetSelfSessionId(const Oid& Id) { m_SelfSessionId = Id; }
+
+	// IWebSocketHandler
+	void OnWebSocketOpen(Ref<WebSocketConnection> Connection) override;
+	void OnWebSocketMessage(WebSocketConnection& Conn, const WebSocketMessage& Msg) override;
+	void OnWebSocketClose(WebSocketConnection& Conn, uint16_t Code, std::string_view Reason) override;
+
 private:
 	struct SessionsStats
 	{
@@ -43,6 +59,7 @@ private:
 
 	void ListSessionsRequest(HttpRouterRequest& Req);
 	void SessionRequest(HttpRouterRequest& Req);
+	void SessionLogRequest(HttpRouterRequest& Req);
 
 	HttpStatusService&		 m_StatusService;
 	HttpStatsService&		 m_StatsService;
@@ -50,6 +67,18 @@ private:
 	SessionsService&		 m_Sessions;
 	SessionsStats			 m_SessionsStats;
 	metrics::OperationTiming m_HttpRequests;
+
+	// WebSocket push
+	RwLock								  m_WsConnectionsLock;
+	std::vector<Ref<WebSocketConnection>> m_WsConnections;
+	asio::steady_timer					  m_PushTimer;
+
+	void BroadcastSessions();
+	void EnqueuePushTimer();
+
+	Oid m_SelfSessionId = Oid::Zero;
+
+	CbObject BuildSessionListResponse();
 };
 
 }  // namespace zen