fix: desktop login centering, BMS app icon, sign out navigation

[iamvirul]

Summary

Three desktop-specific fixes: login card not vertically centered, Flutter default app icon, and sign out not navigating back to login.

Type

• Bug fix

Changes

• Login screen: replaced `Column(spaceBetween)` with `LayoutBuilder + SingleChildScrollView + IntrinsicHeight + Expanded(Center(...))` so the card is always vertically centered on any window size while the footer stays pinned at the bottom
• macOS app icon: generated all required sizes (16, 32, 64, 128, 256, 512, 1024px) from `bms_logo.svg` using `rsvg-convert`, replacing the default Flutter icon in `AppIcon.appiconset`
• Router: changed `appRouterProvider` from auto-dispose to `keepAlive: true`, replaced the recreate-on-auth-change anti-pattern with a `_RouterNotifier + refreshListenable` approach - the GoRouter instance is now stable and re-evaluates its redirect when auth changes, which makes sign out correctly navigate to login

Test plan

• Hot reloaded and tested manually
• Checked on Chrome (flutter run -d chrome)
• No regressions in related screens