docs(mail): add mobile navigation contributor handoff #980

[real-venus]

Closes #980

Summary

Adds a contributor handoff doc for the existing mobile / small-screen navigation surface: the bottom tab bar, sidebar collapse, topbar action cluster, and the responsive switch that drives all three. Documentation-only and scoped to existing app code — no new tool, feature, or product scope, and no code/types/runtime behavior changed.

Changes

• Mobile Navigation Handoff — new src/components/mail/MOBILE_NAVIGATION.md handoff doc for the area; all references link to existing local files, no new architecture invented
• File Map — names the surface's files: BottomNavigation.tsx, Sidebar.tsx, Topbar.tsx, both useIsMobile hooks (hooks/use-mobile.tsx vs lib/use-media-query.ts), and the responsive layout owner routes/index.tsx
• Data Contracts — documents MailFolder, MailFilters/defaultMailFilters, sidebar counts, persisted layout state, and the callbacks-only boundary the nav surfaces respect
• Responsive Behavior — how isMobile coordinates sidebar, content panel, and bottom bar; notes the two same-named useIsMobile hooks and how their breakpoints line up with Tailwind's md
• User-Facing States — bottom-bar active states, sidebar collapsed vs expanded, local-only custom folders, topbar responsive breakpoints, and the account menu
• Safety & Privacy — flags all fake demo identities/counts (Uthaimin, Eve Navarro, quick-action 2/5/9, default folders) and that these surfaces never decrypt, sign, send, or authenticate
• QA Checklist — lightweight per-area checklist for reviewers (768px boundary, bottom-bar visibility/safe-area, sidebar toggle, topbar dropdowns, accessible labels)
• README Link — src/components/mail/README.md links to the new doc for discoverability
• Tests — none added (docs-only); verified tsc --noEmit clean, 439/439 unit tests pass, vite build succeeds, 0 new lint errors