Add state management, WebSocket polling, html.js templating, and refactor pages

- lib/state.py: per-subsystem collectors with versioned state store
- daemon/server.py: state refresh on request, batch routing updates
- webui/static/hoover/html.js: new html tag template helper via htm.js
- webui/static/hoover/websocket.js: real-time state change notifications
- webui/static/hoover/vdom.js: VDOM improvements for keyed diff
- All frontend pages refactored to use html templates
- Add tests for state management and polling
- Update docs and AGENTS.md

docs/api.md

@@ -1502,4 +1502,20 @@ GET /api/logs/app
 
 Return recent application log entries as rendered HTML.
 
-**Response:** HTML fragment of `<div class="log-line">` elements.
+**Response:** HTML fragment of `<div class="log-line">` elements.
+
+---
+
+## WebSocket Protocol
+
+The daemon exposes a WebSocket at `/ws` (port 9091) for real-time state change notifications. On connect, the server sends:
+
+```json
+{"type": "init", "versions": {"firewall": 0, "dnsmasq": 0, ...}}
+```
+
+### Message Types
+
+- **`versions`** — Structural state change. `updated` contains subsystem names whose version counters changed. Triggers full re-fetch.
+- **`tick`** — Volatile-only change (stats, counters, DHCP IPs). `subsystems` contains affected subsystem names. Triggers lightweight per-subsystem re-fetch.
+- **`notify`** — Single-topic notification. `topic` is the subsystem name.