fix(scripts): rewrite PWA installer as real installer (systemd + venv + pacman checks)

- install_mobile_pwa.sh: full installer with:
  - System dependency checks (python3, venv, systemctl, curl)
  - Python venv creation in ~/.omni-mobile-venv
  - Core deps (flask, opencv-python-headless, pydantic, Pillow, numpy)
  - systemd user service (omni-mobile-pwa.service) with proper
    Environment= on separate lines, Restart=on-failure
  - Manual launch fallback when systemd --user unavailable (containers)
  - Health check (GET /health with retry loop)
  - Desktop shortcut (.desktop file) creation
  - Idempotent: re-runs safely over existing installation

- uninstall_mobile_pwa.sh: complete uninstaller with:
  - systemd service stop + disable + file removal
  - venv removal (~/.omni-mobile-venv)
  - Desktop shortcut + icon cleanup
  - Data preservation by default (data/ kept)
  - --purge-data flag with double confirmation (interactive YES)
  - --yes flag for non-interactive use
  - Manual process kill fallback when systemctl unavailable

Tested:
  - install: /health returns {status:ok, app_services_loaded:true,
    learning_loop_loaded:true}
  - uninstall: data/ preserved, all artifacts removed
  - --purge-data with --yes: data/ deleted
  - Idempotent re-install: works over existing install
  - Mission 2 (check_appimage_freshness.py): still exits 2 (NO_BUILD)

Author: Z User