refactor(ui): load app.js as an ES module (R3a) (#876)

One attribute. #871/#872/#874/#875 exist to make this line safe.

app.js's 385 top-level `function` declarations stop being implicit `window`
properties: 87 stay reachable via the explicit contract (#874's Object.assign
block + the 47 pre-existing `window.X = X` assignments), and 298 become
module-private. Verified NO unexposed name is read from outside app.js.

Strict mode (modules are always strict) checked ahead of the flip: app.js parses
clean as `sourceType: module` (no octal, dup params, `with`), and has no implicit
globals, no `eval`/`new Function`, no top-level `this`. `registerShortcut` is
called bare at 15 top-level sites but is assigned at `window.registerShortcut`
(app.js:10387) before its first call (10648), and a bare identifier in a module
still resolves through the global object — verified `typeof
window.registerShortcut === 'function'` in the browser.

HARD GATE — app.js IS the plugin loader:
  - /api/plugins script_type passthrough: editor/stems/studio = "module"
  - /api/plugins/stems/src/main.js -> 200; conditional GET -> 304 (live-edit ETag)
  - deep graph: stems/src/transport.js, editor/src/state.js -> 200
  - window.loadPlugins present; 5 plugin screens mount; the 3 migrated plugins
    injected as <script type="module">
  - 37 capability participants, 14 compatibility shims, bus + capabilities v1

Every one of the shell's 336 inline handlers resolves on window under module
scope, and the A-Z rail / pagination execute 6/6 with no ReferenceError. A/B
against origin/main: the ONLY unresolvable handler is `editorToggleStemMixer`,
which is equally broken on main (a dead handler in the editor plugin — not
defined anywhere in its source; pre-existing, flagged separately).

Codex preflight raised a [P1] claiming restartCurrentSong / requestExitSong /
editRegionInEditor / returnToEditorFromHighway would ReferenceError — FALSE
POSITIVE. It scanned only #874's new Object.assign block and missed app.js's 47
scattered `window.X = X` assignments; all four are at app.js:7086/7204/8492/8511
and all four resolve as `function` in the browser with app.js loaded as a module.

pytest 2396, node 1032/1032, ESLint 0 errors, tailwind-fresh clean.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

static/v3/index.html

@@ -1244,7 +1244,7 @@
     <script defer src="/static/highway.js"></script>
     <script defer src="/static/vendor/lottie.min.js"></script>
     <script defer src="/static/lottie-api.js"></script>
-    <script defer src="/static/app.js"></script>
+    <script type="module" src="/static/app.js"></script>
     <script defer src="/static/audio-mixer.js"></script>
     <script defer src="/static/vendor/shepherd.min.js"></script>
     <script defer src="/static/tour-engine.js"></script>