dashboard: rename src/app.js → src/tabs.js (closes #406)

#406 was the JS-split refactor: pull shared helpers into
common.js (step 1), pull the flow-only IIFEs into flow.js
(step 2), then rename the legacy combined entry from app.js
to tabs.js (step 3 — this commit) to reflect that the bundle
now owns the dashboard tabs surface only.

What moved:
- `frontend/packages/dashboard/src/app.js` → `tabs.js`
- `build.mjs` entry: `src('app.js')` → `src('tabs.js')`;
  output is now `static/tabs.js`
- `index.html` `<script>` src: `/static/app.js` →
  `/static/tabs.js`; the FL0W-section comment retouched
- `flow.html` reference from `/static/app.js`-as-tabs to
  `/static/tabs.js`; notif + offscreen-inbox comments rewired
  to point at the correct owners (common.js for NOTIF,
  flow.js for renderInbox)
- `common.js`, `flow.js`, `tabs.js` headers: scrub stale
  "app.js" references; document that #406 steps 2 + 3 are
  done and both pages import directly from common.js
- `dashboard.css` comments: every "by app.js" → "by tabs.js"
- `docs/web-ui.md`, `docs/conventions.md`: legacy
  `assets/app.js` → `assets/tabs.js` (the path prefix is
  itself stale from a much-earlier rename, but consistent
  with the rest of those docs)
- `CLAUDE.md` file map: refresh the dashboard package's
  src/ and dist/ layouts to reflect the post-split shape
  (tabs.js, flow.js, common.js, stream-worker.js)

What did NOT move:
- `frontend/packages/agent/src/app.js` (per-agent UI entry)
  stays named `app.js` — it's a separate package, has only
  one entry point, no split happened there
- The two "no-op when target absent" guards in renderContainers
  and renderQuestions are kept as belt-and-suspenders for any
  future page that adds tabs.js without the corresponding
  sections; comments updated to note this rather than the
  pre-split flow.html reason

Functional behaviour identical; this is a pure rename + comment
sweep.

frontend/packages/dashboard/src/flow.html

@@ -41,7 +41,8 @@
       </a>
 
       <!-- Notif controls cohabit with the tabs (always-on chrome).
-           Same IDs as on the dashboard so app.js's NOTIF binding
+           Same IDs as on the dashboard so the shared NOTIF binding
+           (from common.js, imported by both tabs.js and flow.js)
            picks them up unchanged. -->
       <div id="notif-row" class="notif-row">
         <button type="button" id="notif-enable" class="btn btn-notif" hidden>🔔 enable notifications</button>
@@ -84,7 +85,7 @@
     </div>
   </footer>
 
-  <!-- Inbox rendered offscreen — kept in the DOM so app.js's
+  <!-- Inbox rendered offscreen — kept in the DOM so flow.js's
        renderInbox keeps working unchanged. The pill click handler
        opens the side panel which displays a clone of the list. The
        legacy section heading would otherwise be visible; hidden
@@ -113,7 +114,7 @@
   <!-- Flow-specific bundle (#406 step 2). Contains the broker
        terminal init, the operator-inbox derived store, the inbox
        pill flyout, and the @-mention composer. Tab renderers etc.
-       live in `/static/app.js` which /flow.html doesn't need. -->
+       live in `/static/tabs.js` which /flow.html doesn't load. -->
   <script type="module" src="/static/flow.js" defer></script>
 </body>
 </html>