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/index.html

@@ -173,7 +173,7 @@
 
     <!-- FL0W: lives on its own page now (`/flow.html`). The
          message-flow + inbox + compose DOM only exists there — when
-         app.js boots on this page the corresponding renderers
+         tabs.js boots on this page the corresponding renderers
          no-op silently (each guard is `if (!el) return`). -->
 
   </main>
@@ -216,9 +216,11 @@
             title="clear selection (esc)">✕ clear</button>
   </div>
 
-  <!-- Single bundled entry. esbuild folds @hive/shared/terminal.js and
-       the marked npm package into app.js; load order is preserved by
-       the module bundler. -->
-  <script type="module" src="/static/app.js" defer></script>
+  <!-- Single bundled entry (#406 step 3 — renamed from app.js to
+       tabs.js since this bundle is the dashboard *tabs* surface only;
+       flow.html has its own flow.js bundle). esbuild folds
+       @hive/shared/terminal.js and the marked npm package into
+       tabs.js; load order is preserved by the module bundler. -->
+  <script type="module" src="/static/tabs.js" defer></script>
 </body>
 </html>