refactor: Migrate navigation to nav3

[ovitrif]

Resolves #480

Description

This PR migrates the entire navigation system from legacy Compose Navigation (NavController) to the newly stable Navigation 3 (Nav3).

Key changes:

• Updated all navigation usage with Nav3 primitives
• Moved all navigation code to its dedicated nav package
• Moved all type-safe route definitions to Routes.kt sealed interface
• Introduced Navigator class centralising navigation logic
• Added SheetSceneStrategy for custom bottom sheets
• Replaced material3 BottomSheetScaffold with custom sheet implementation
• Entry handlers in nav/entries/ for all screen and sheet routes
• Removed legacy code and old sheet implementations
• Updated 55+ screens from NavController to Navigator pattern
• Net reduction of +3k lines of code
• TabBar now slides out below the viewport when showing sheets or non-wallet screens

Preview

nav3.mp4

QA Notes

• Verify all screen navigation works (back buttons, forward navigation)
• Test all bottom sheets open and close correctly
• Verify sheet dismiss behavior (swipe down, back button, overlay tap)
• Test deep link handling
• Test timed sheets (backup reminder, app update, etc.)
• Verify drawer menu navigation
• Test QR scanner navigation flow
• Verify settings screens navigation hierarchy
• Verify app with pin on launch enabled
• Verify auto-read clipboard enabled

[ovitrif]

re-drafted because it's not yet ready