hive-dashboard: remove the MATRIX credentials tab and its login route

The CR3D3NTIALS page's MATRIX tab was the only caller of
`POST /api/matrix-account-login` (provision/log in an external matrix
account through the hive) and `GET /api/matrix-accounts` (its account
list). External matrix accounts are linked from the swarm UI now
(`LinkMatrixAccountForm` -> swarm-controller), so the hive-side UI and
both routes go. `priv_client::restart_matrix_daemon` had no other caller
and goes with them.

Already-provisioned credentials keep working: the `matrix-token-<name>`
files and `matrix-account-<name>.json` sidecars the old route wrote are
still discovered by hive-matrix-mcp (`accounts::configured` ->
`discover_token_accounts`), the `matrix-token*` path unit still re-fires
the daemon, and `WriteAgentMatrixToken` stays for the swarm credential
worker. Removing that usage waits on moving the existing creds to
swarm level.

The GITHUB tab is the credentials page's default tab now.

Refs #4348

frontend/packages/dashboard/src/index.html

@@ -96,7 +96,7 @@
               <span class="home-tile-label">Credentials</span>
             </span>
             <span class="home-tile-desc"
-              >provision per-agent matrix + github accounts</span
+              >provision per-agent github + forge accounts</span
             >
           </a>