refactor(frontend): extract mobile nav and header projection from app.ts into a bounded-context module

[cristim]

Problem

frontend/src/app.ts is well past the 500-line ceiling stated in CLAUDE.md
("Keep files under 500 lines"), and the mobile navigation / header projection
behaviour is the largest self-contained chunk in it.

Line counts:

Revision	frontend/src/app.ts
before PR LeanerCloud/cloud-commitments-cli#1877 (b83c5d14e)	802
PR LeanerCloud/cloud-commitments-cli#1877 head	883

The violation predates PR LeanerCloud/cloud-commitments-cli#1877. At 802 lines the file was already 60%
over the ceiling. PR LeanerCloud/cloud-commitments-cli#1877 added 81 lines to it: 63 in the fix for LeanerCloud/cloud-commitments-cli#1779 plus
18 in the accessibility follow-up on the widening path.

CodeRabbit raised the extraction on PR LeanerCloud/cloud-commitments-cli#1877 and it was declined there on
scope grounds: it is a restructuring of a file shared by most of the frontend,
unrelated to the p1 bug that PR fixes, and bundling it would have both blown
the PR's blast radius and buried the actual fix.

Proposal

Move the mobile-nav and header-projection behaviour out of app.ts into its
own bounded-context module (for example frontend/src/nav/mobile-nav.ts)
exposing a typed public API rather than loose exported functions.

Symbols to move (all currently in frontend/src/app.ts):

• MOBILE_NAV_QUERY, the (max-width: 768px) breakpoint constant
• DRAWER_PROJECTED_IDS
• syncHeaderPlacement(isNarrow: boolean)
• setupMobileNav() and its inner openDrawer / closeDrawer /
  releaseDrawerForDesktop transitions

Blast radius

Small, which is what makes this worth doing as its own change:

• frontend/src/__tests__/mobile-nav.test.ts is the only importer
  (import { setupMobileNav, syncHeaderPlacement } from '../app'). The jest
  coverage already lives in its own file, so the test move is a re-point of
  one import.
• frontend/tests-e2e/mobile-header-drawer.spec.ts drives the behaviour
  through the browser and needs no change.
• Three comments name the current home and should be updated in the same
  change: frontend/src/index.html (the #sidebar-extras slot),
  frontend/src/styles/responsive.css lines 76 and 178.

Acceptance

• frontend/src/app.ts drops below 500 lines, or the remaining excess is
  identified as a separate follow-up with its own extraction target.
• The new module exports a typed API; no behaviour change.
• npm test, npm run lint, tsc --noEmit and the full Playwright suite
  stay green, with the mobile-nav jest and e2e coverage unchanged in substance.