feat: add distro mount configurations

[mohit-kota]

Closes #3

Summary

• replaces hardcoded-only Proot bindings with distro-level mount configuration profiles
• keeps uDroid's current six bindings as restorable defaults
• supports enabling/disabling defaults plus adding, editing, disabling, and deleting custom mappings
• creates an independent distro filesystem for each named configuration
• lists configurations from the source distro with launch, edit, and delete actions
• applies the saved profile consistently to terminal, application, and desktop launches
• preserves intentionally invalid profiles and reports startup failures gracefully

Flow

1. Open mount configurations from a distro	2. View and manage attached configurations

3. Name a configuration	4. Configure default and custom mappings

5. Open the attached distro	6. Inspect its saved mount summary

Storage and isolation

Each installed system owns files/linux-systems/<system-id>/mounts.json. Generated configurations record their source system ID but receive a separate rootfs directory and independent profile file.

Verification

./gradlew :app:testDebugUnitTest :app:lintDebug :app:assembleDebug
BUILD SUCCESSFUL

The complete architecture, storage model, validation rules, and rollout notes are documented in docs/CONFIGURABLE_PROOT_MOUNTS_PLAN.md.