feat(app): add native mobile navigation

[ctwhome]

Summary

• Adds native-feeling hierarchical mobile swipe navigation from chat → Sessions → Projects, plus direction-correct drawer dismissal.
• Tracks the finger with horizontal/vertical intent locking, distance/velocity thresholds, snap-back, reduced-motion behavior, click suppression, safe-edge and interactive-surface exclusions.
• Adds route-authoritative browser/Android Back history, durable ordinary-launch restoration, explicit deep-link precedence, stale-target recovery, and dirty-edit protection.
• Restores focus to the matching Projects/Sessions trigger and keeps visible/keyboard/screen-reader fallbacks.
• Compacts the composer model label to the actual model identity and repairs the 320/390px composer geometry while preserving 44px targets.

Verification

• bun test — 330 passed, 4 opt-in integration tests skipped
• bun run --cwd app check — 0 errors, 0 warnings
• production build — passed (existing chunk-size warning only)
• bun run --cwd app test:e2e — 46 passed
• focused dirty Back, slow-drag, reduced-motion click suppression, stale Session, and focus restoration coverage — passed
• changed-file Prettier and git diff --check — passed
• independent blocker review — CLEAN after all five findings were repaired

Device evidence

Automated touch/mobile coverage includes 320px and 390px. A physical Android-device PWA pass remains part of final installed-PWA verification with #85; this PR does not claim hardware evidence.

Closes #83