fix(web-ui): security hardening, SPA nav, and modularization

Ship the in-progress ES-module refactor of the web-ui (new static/modules/
layout, Usage/Settings pages, uplot-based dashboard) alongside a round of
security and UX fixes:

- main.go: add CSP + X-Frame-Options: DENY + X-Content-Type-Options:
  nosniff + Referrer-Policy middleware on every response; WS CheckOrigin
  now requires Origin host to match Host (blocks cross-site WebSocket
  hijacking); upgrade client before dialing upstream so origin check
  runs first; fatal on unparseable AGENTMON_QUERY_BASE.
- app.js: delegated click handler intercepts same-origin <a> clicks for
  SPA navigation (prev. every nav link caused a full page reload,
  dropping WS + in-memory state); delegated .copy-btn[data-copy]
  handler replaces inline onclick=; removed window.navigate /
  window.copyToClipboard globals and the duplicated handleGlobalSearch.
- modules/nav-signal.js: per-route AbortController so in-flight fetches
  are cancelled when the user navigates away, preventing stale toasts
  and wasted renders.
- modules/api.js: honours the nav signal by default; AbortError is
  silent.
- modules/router.js: resets the nav controller on every route; dropped
  the fixed 80ms transition delay; breadcrumbs no longer emit inline
  onclick= (delegated handler picks them up).
- modules/utils.js: renderCopyButton emits data-copy=\"...\" instead of
  nesting a JS string inside an HTML attribute — fixes an XSS where
  values containing ' broke out via &#39; decoding.

Verified: go build clean; `node --check` clean on all modified modules;
manual curl probes confirm security headers present on every response
and WS upgrade returns 403 for cross-origin/missing Origin while 101
for same-origin.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/web-ui/static/modules/pages/settings.js

@@ -0,0 +1,49 @@
+import { app } from '../router.js';
+import { showToast } from '../api.js';
+
+export async function renderSettings() {
+  app.innerHTML = `
+    <div class="page-header">
+      <h2>Settings</h2>
+    </div>
+
+    <div class="settings-section">
+      <h3 class="settings-section-title">Data Retention</h3>
+      <p class="settings-section-desc">Delete events older than the specified number of days. This runs automatically every 24 hours. Currently configured via <code>RETENTION_DAYS</code> environment variable (default: 30 days).</p>
+      <div class="settings-row">
+        <label class="settings-label" for="retention-days">Purge events older than</label>
+        <div class="settings-input-group">
+          <input type="number" id="retention-days" class="settings-input" min="1" max="365" value="30">
+          <span class="settings-input-suffix">days</span>
+          <button class="settings-btn" id="run-retention-btn" type="button">Run Now</button>
+        </div>
+      </div>
+      <div id="retention-result" class="settings-result"></div>
+    </div>
+  `;
+
+  document.getElementById('run-retention-btn')?.addEventListener('click', async () => {
+    const days = parseInt(document.getElementById('retention-days').value, 10);
+    if (!days || days < 1) { showToast('Enter a valid number of days', 'error'); return; }
+    const btn = document.getElementById('run-retention-btn');
+    btn.disabled = true;
+    btn.textContent = 'Running…';
+    try {
+      const resp = await fetch('/api/v1/admin/retention', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ days }),
+      });
+      const data = await resp.json();
+      if (!resp.ok) throw new Error(data.error || 'Request failed');
+      const result = document.getElementById('retention-result');
+      if (result) result.innerHTML = `<span class="settings-result-ok">Deleted ${data.deleted} events older than ${new Date(data.cutoff).toLocaleDateString()}.</span>`;
+      showToast(`Deleted ${data.deleted} events`, 'success');
+    } catch (e) {
+      showToast('Retention failed: ' + e.message, 'error');
+    } finally {
+      btn.disabled = false;
+      btn.textContent = 'Run Now';
+    }
+  });
+}