agent UI: relative paths for all assets/api/ws so the page works under any nginx prefix (#14)

Per mara on #14: 'make agent page not assume root path, links / api
calls need to be relative'. atlas's nginx side (#15) will mount the
per-agent UI at a prefix like /agent/<name>/ instead of its own
port; for the page to keep working under that prefix, every
in-page reference needs to resolve document-relative rather than
root-anchored.

Converted in this pass:
- HTML <link>/<script>/<img>/<a> hrefs in index.html, stats.html,
  screen.html: '/icon' → 'icon', '/static/agent.css' →
  'static/agent.css', back links '/' → './'.
- app.js fetch() targets ('/api/state' → 'api/state', /api/cancel,
  /api/loose-ends, etc.), form actions ('/login/start', '/send'),
  EventSource urls ('/events/stream', '/events/history').
- stats.js fetch() targets.
- screen.html WebSocket URL: was hardcoded as
  ws(s)://host/screen/ws; now derived from document.baseURI via
  new URL('screen/ws', document.baseURI) so the gateway prefix
  flows through.

Slash-command labels (/cancel, /compact, …) and the dashboard-port
link (different port, intentionally absolute) intentionally
untouched.

Added a new 'Per-agent relative paths' section to docs/web-ui.md
covering the rationale + the trailing-slash gotcha (sub-pages like
/stats must NOT have a trailing slash, or 'static/app.js' resolves
under /stats/ instead of replacing the segment).

Functional code unchanged; build clean. Damocles + atlas can
proceed with the backend / nginx side without depending on this
landing first, but once both ship the agent page works under the
gateway-prefixed URL without further changes.

refs #14

frontend/packages/agent/src/stats.js

@@ -306,7 +306,7 @@ window.Chart = Chart;
 
   async function loadStats() {
     try {
-      const resp = await fetch('/api/stats?window=' + encodeURIComponent(currentWindow));
+      const resp = await fetch('api/stats?window=' + encodeURIComponent(currentWindow));
       if (!resp.ok) throw new Error('http ' + resp.status);
       const snap = await resp.json();
       render(snap);
@@ -317,7 +317,7 @@ window.Chart = Chart;
 
   async function loadIdentity() {
     try {
-      const resp = await fetch('/api/state');
+      const resp = await fetch('api/state');
       if (!resp.ok) return;
       const s = await resp.json();
       document.title = 'stats · ' + s.label;