feat(navigation): rebuild the bottom bar and rail on M3 Expressive

[A-EDev]

Summary

Rebuilds the bottom navigation bar and the tablet side rail on Material 3 Expressive.

• Bottom bar now uses ShortNavigationBar, with the same background colour as before. Six or seven tabs go behind a "More" menu.
• Side rail items are centred vertically. On large windows the rail can be expanded from its top menu button.
• Fixes the bar appearing on screens like Settings and Playlists after collapsing the video or music mini player.
• Adds a filled Shorts icon for the selected tab.
• Tapping Library, Explore or Search again scrolls back to the top.
• The tab order setting is now drag-to-reorder.
• Splits ContentSettingsScreen into section files and slims down FlowApp, with no visual change.

Related issue

closes #805

Change type

• Bug fix
• Feature
• Refactor or maintenance
• Build, packaging, or CI
• Documentation

Validation

• ./gradlew :app:assembleGithubDebug
• ./gradlew :app:testGithubDebugUnitTest
• I ran any additional flavor-specific build or test tasks affected by this change. (:app:compileFossDebugKotlin, ktlintCheck)
• I manually tested the affected behavior on an Android device or emulator.

Test device and Android version: Xiaomi POCO X6 Pro, Android 16, HyperOS 3.0.9.0

Screenshots or recordings

Risk and compatibility

Saved tab order and default tab settings keep their existing values, so no preference migration is needed.

• The change does not introduce secrets, private data, or unexpected telemetry.
• New user-facing text uses Android string resources.
• Dependency and lockfile changes are intentional and limited to this PR.
• Room schema changes include the required version bump and migration, or this PR does not change the Room schema.
• Breaking changes and upgrade steps are clearly documented.

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