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

hive-c0re/src/priv_client.rs

@@ -445,17 +445,6 @@ pub async fn delete_agent_extra_forge_account(agent_name: &str, label: &str) ->
     .await?)
 }
 
-/// Restart `hive-matrix-daemon.service` inside an agent container via
-/// `systemctl --machine=h-<agent_name> restart hive-matrix-daemon.service`.
-/// Non-fatal: callers should handle errors gracefully — if the container is
-/// not running the restart will fail (the unit starts naturally on next boot).
-pub async fn restart_matrix_daemon(agent_name: &str) -> Result<()> {
-    ok(call(&PrivRequest::RestartMatrixDaemon {
-        agent_name: agent_name.to_owned(),
-    })
-    .await?)
-}
-
 /// Register the hive-ci Forgejo Actions runner: hand the freshly-minted
 /// registration token to hive-priv, which writes it to the host-side
 /// `/run/hive-ci/runner-token` env-file and restarts the in-container runner.