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, 7 insertions, 22 deletions

diff --git a/src/zenhttp/monitoring/httpstats.cpp b/src/zenhttp/monitoring/httpstats.cpp
index 3877215a8..283cedca7 100644
--- a/src/zenhttp/monitoring/httpstats.cpp
+++ b/src/zenhttp/monitoring/httpstats.cpp
@@ -189,11 +189,8 @@ HttpStatsService::BroadcastStats()
 		return;
 	}
 
-	// Collect stats from all providers
-	ExtendableStringBuilder<4096> JsonBuilder;
-	JsonBuilder.Append("{");
-
-	bool First = true;
+	// Collect stats from all providers into a single CBO object, then convert to JSON
+	CbObjectWriter Writer;
 	{
 		RwLock::SharedLockScope _(m_Lock);
 		for (auto& [Id, Provider] : m_Providers)
@@ -203,27 +200,15 @@ HttpStatsService::BroadcastStats()
 			{
 				continue;
 			}
-
-			if (!First)
-			{
-				JsonBuilder.Append(",");
-			}
-			First = false;
-
-			// Emit as "provider_id": { ... }
-			JsonBuilder.Append("\"");
-			JsonBuilder.Append(Id);
-			JsonBuilder.Append("\":");
-
-			ExtendableStringBuilder<2048> StatsJson;
-			Stats.ToJson(StatsJson);
-			JsonBuilder.Append(StatsJson.ToView());
+			Writer.AddObject(Id, Stats);
 		}
 	}
 
-	JsonBuilder.Append("}");
-
+	CbObject					  Payload = Writer.Save();
+	ExtendableStringBuilder<4096> JsonBuilder;
+	Payload.ToJson(JsonBuilder);
 	std::string_view Json = JsonBuilder.ToView();
+
 	for (auto& Conn : Connections)
 	{
 		if (Conn->IsOpen())