dashboard: point frontend at the /api/* backend routes

Phase 2 of the dashboard route consolidation. The backend now double-registers
every bare top-level route (approve/deny/kill/restart/start/rebuild/destroy/
update-all/answer-question/cancel-question/purge-tombstone/matrix-account-login/
cancel-reminder/retry-reminder/request-spawn/op-send/meta-update/dashboard-stream/
dashboard-history) at an additional /api/<same> path. Switch every dashboard-pkg
fetch / EventSource / form action to the /api/ form so "backend = /api/*" holds
on the frontend side too.

Bare paths still answer, so this is independently deployable; once it ships the
backend can drop the bare registrations (phase 3). webhook/knowledge stays a
distinct webhook prefix (forge-driven, no SPA caller). app.js's /rebuild,/start
are the agent harness UI (different server) and are untouched.

Part of #1846 (phase 2).

frontend/packages/dashboard/src/matrix-accounts.js

@@ -139,7 +139,7 @@ async function submitLogin(e) {
   btn.textContent = 'logging in…';
 
   try {
-    const resp = await fetch('/matrix-account-login', {
+    const resp = await fetch('/api/matrix-account-login', {
       method: 'POST',
       headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
       body: new URLSearchParams(fd),