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.

docs/web-ui.md

@@ -15,7 +15,7 @@ This doc has been split for readability. Pick the section you need:
 - **[Dashboard layout](web-ui/dashboard.md)** — tab contents
   (SW4RM, Y3R C4LL, P3RM1SS10NS, SCH3DUL3S) + standalone pages
   (C0R3 `/core.html`, BU1LDS `/builds.html`, L0GS `/logs.html`,
-  M4TR1X ACC0UNTS `/matrix-accounts.html`), container row,
+  CR3D3NTIALS `/credentials.html`), container row,
   topology tree, selection bar, approval card, browser
   notifications, dashboard endpoints + event channel.
 - **[Per-agent page](web-ui/agent.md)** — header, main terminal,