M3.1: Build welcome app (first-run experience + mirror toggle)

[mshafiee]

Acceptance criteria

• First-run experience launches on initial login
• Keyboard layout selection (US us / Persian ir per ISIRI 9147)
• APT repo opt-in (register Zurvan custom repo + key)
• Links to docs and community resources
• Debian mirror toggle: switch main Debian sources between deb.debian.org and trusted local university/ISP mirrors (Iranserver, Tabriz University, etc.)

Scope

Per project-next-steps.md M3.1 "Welcome App" and distribution-update-strategy.md §4 (upstream mirror optimization):

• First-run experience.
• Keyboard layout selection.
• APT repo opt-in.
• Links to docs and community.
• Local-mirror toggle in the Welcome Screen.

Dependencies

• M1.2 APT repo + public.key available
• M3.3 installer GPG key auto-import

Notes

Repo: welcome-app. The mirror toggle is an intentional feature for users on restricted/throttled Iranian networks (see AGENTS.md — not a bug).