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, 24 insertions, 57 deletions

diff --git a/src/zenserver/frontend/html/pages/start.js b/src/zenserver/frontend/html/pages/start.js
index 580045060..df70ea2f4 100644
--- a/src/zenserver/frontend/html/pages/start.js
+++ b/src/zenserver/frontend/html/pages/start.js
@@ -17,6 +17,29 @@ export class Page extends ZenPage
 		const api_data = await new Fetcher().resource("/api/").json();
 		const available = new Set((api_data.services || []).map(s => s.base_uri));
 
+		// stats tiles
+		const safe_lookup = (obj, path, pretty=undefined) => {
+			const ret = path.split(".").reduce((a,b) => a && a[b], obj);
+			if (ret === undefined) return undefined;
+			return pretty ? pretty(ret) : ret;
+		};
+
+		var section = this.add_section("Activity");
+		section.tag().classify("dropall").text("metrics dashboard →").on_click(() => {
+			window.location = "?page=metrics";
+		});
+
+		var providers_data = await new Fetcher().resource("stats").json();
+		var provider_list = providers_data["providers"] || [];
+		var all_stats = {};
+		await Promise.all(provider_list.map(async (provider) => {
+			all_stats[provider] = await new Fetcher().resource("stats", provider).json();
+		}));
+
+		this._stats_grid = section.tag().classify("grid").classify("stats-tiles");
+		this._safe_lookup = safe_lookup;
+		this._render_stats(all_stats);
+
 		// project list
 		var project_table = null;
 		if (available.has("/prj/"))
@@ -102,29 +125,6 @@ export class Page extends ZenPage
 			}
 		}
 
-		// stats tiles
-		const safe_lookup = (obj, path, pretty=undefined) => {
-			const ret = path.split(".").reduce((a,b) => a && a[b], obj);
-			if (ret === undefined) return undefined;
-			return pretty ? pretty(ret) : ret;
-		};
-
-		var section = this.add_section("Stats");
-		section.tag().classify("dropall").text("metrics dashboard →").on_click(() => {
-			window.location = "?page=metrics";
-		});
-
-		var providers_data = await new Fetcher().resource("stats").json();
-		var provider_list = providers_data["providers"] || [];
-		var all_stats = {};
-		await Promise.all(provider_list.map(async (provider) => {
-			all_stats[provider] = await new Fetcher().resource("stats", provider).json();
-		}));
-
-		this._stats_grid = section.tag().classify("grid").classify("stats-tiles");
-		this._safe_lookup = safe_lookup;
-		this._render_stats(all_stats);
-
 		// version
 		var ver_tag = this.tag().id("version");
 		var version = new Fetcher().resource("health", "version");
@@ -135,40 +135,7 @@ export class Page extends ZenPage
 		this._cache_table = cache_table;
 
 		// WebSocket for live stats updates
-		this._connect_stats_ws();
-	}
-
-	_connect_stats_ws()
-	{
-		try
-		{
-			const proto = location.protocol === "https:" ? "wss:" : "ws:";
-			const ws = new WebSocket(`${proto}//${location.host}/stats`);
-
-			try { this._ws_paused = localStorage.getItem("zen-ws-paused") === "true"; } catch (e) { this._ws_paused = false; }
-			document.addEventListener("zen-ws-toggle", (e) => {
-				this._ws_paused = e.detail.paused;
-			});
-
-			ws.onmessage = (ev) => {
-				if (this._ws_paused)
-				{
-					return;
-				}
-				try
-				{
-					const all_stats = JSON.parse(ev.data);
-					this._render_stats(all_stats);
-				}
-				catch (e) { /* ignore parse errors */ }
-			};
-
-			ws.onclose = () => { this._stats_ws = null; };
-			ws.onerror = () => { ws.close(); };
-
-			this._stats_ws = ws;
-		}
-		catch (e) { /* WebSocket not available */ }
+		this.connect_stats_ws((all_stats) => this._render_stats(all_stats));
 	}
 
 	_render_stats(all_stats)