agent page: link to the agent's forge profile

Add a '⬡ forge ↗' link to the per-agent page's meta row, next to
the stats + screen links. It opens the agent's Forgejo profile
(http://<host>:3000/<label> — the per-agent forge user is named
after the agent) in a new tab.

- web_ui.rs: StateSnapshot gains forge_present, true when the
  agent's forge-token file exists in the state dir (same signal
  that tells the agent it has a forge account).
- index.html / app.js: hidden link, shown + href-filled when
  forge_present, mirroring the existing gui_enabled/screen-link
  pattern. Host comes from window.location so it works off
  whatever host the page is served from.

closes #185

docs/web-ui.md

@@ -358,6 +358,10 @@ Layout, top to bottom:
 
 - Banner (gradient shimmer while state=thinking).
 - Title with `↑ DASHB04RD` back-link (new tab) + `↻ R3BU1LD`.
+- Meta links row: `📊 stats →`, `🖥 screen →` (shown when
+  `gui_enabled`), and `⬡ forge ↗` (shown when `forge_present` —
+  links to the agent's Forgejo profile at `<host>:3000/<label>`,
+  new tab).
 - Status section: empty when online (alive-badge in the state
   row carries the signal), populated with the login form /
   OAuth URL when `status` is `needs_login_*`.