feat: rename matrix-accounts page to credentials, add github PAT tab

Part 2 of hyperhive#1970 (backend/CLI landed in PR #2378). Renames
/matrix-accounts.html -> /credentials.html and restructures it with a
sub-tab strip (MATRIX / GITHUB), reusing the shared @hive/shared/tabs.js
tab strip already used by /logs.html.

MATRIX tab carries over the existing account list + login form
unchanged. GITHUB tab adds a single-PAT provisioning form: status line
(present/absent, read from GET /api/github-account), a security-warning
banner (dedicated bot account + minimally-scoped token), a link to
generate a PAT at github.com/settings/tokens, and a paste-token form
posting to POST /api/github-account. Both tabs share one agent picker.

Updated docs/web-ui.md + docs/web-ui/dashboard.md to describe the new
page shape, and the H0M3 hub tile (index.html) to point at the renamed
page.

frontend/packages/dashboard/src/index.html

@@ -91,12 +91,12 @@
         <span class="home-tile-desc">kept state · container load</span>
       </a>
 
-      <a class="home-tile" href="/matrix-accounts.html">
+      <a class="home-tile" href="/credentials.html">
         <span class="home-tile-head">
           <span class="home-tile-icon" aria-hidden="true">🔑</span>
-          <span class="home-tile-label">Matrix accounts</span>
+          <span class="home-tile-label">Credentials</span>
         </span>
-        <span class="home-tile-desc">provision · log in · store per-agent matrix tokens</span>
+        <span class="home-tile-desc">provision per-agent matrix + github accounts</span>
       </a>
 
       <!-- Forge tile: hidden until home.js confirms the hive-forge