Refactor settings storage to Preferences DataStore to remove Protobuf

[davidjiagoogle]

This PR refactors settings storage from Proto DataStore to Preferences DataStore. This allows us to completely remove the com.google.protobuf dependency and its generated Java code, saving ~134 KB of bytecode footprint in platform environments where dependencies are linked dynamically.

[gemini-code-assist]

Code Review

This pull request migrates the application's settings storage from Proto DataStore to Preferences DataStore, removing all Protobuf dependencies, plugins, and schema files. Model enums and DebugSettings have been updated to remove proto conversion methods, with DebugSettings now using a custom string-based serialization format. LocalSettingsRepository has been refactored to read and write settings using Preferences keys. The review feedback highlights critical robustness issues where unsafe string parsing (using valueOf directly on stored preferences or parsing DebugSettings without validation) could lead to runtime crashes (e.g., IllegalArgumentException or NumberFormatException) if the data is corrupted or malformed. Implementing safe parsing helpers with fallback defaults is highly recommended to ensure app stability.

[temcguir]

Since we are replacing Protobuf with a custom, hand-rolled string parser, we need unit tests to ensure this logic is robust. String parsing based on delimiters (:, ;, ,) and specific string formats (like SolidColor(...)) is prone to regressions if the model changes in the future.

Could you please add a DebugSettingsTest.kt in the core:model module? It should verify that parseFromString(encodeAsString()) correctly rebuilds the object. Specifically, we should test:

1. Default values (all fields empty/off).
2. Simple TestPattern variants (like ColorBars).
3. The complex TestPattern.SolidColor variant to ensure the regex/splitting of the 4 color channels works correctly.
4. Edge cases like missing delimiters or malformed strings (to ensure it falls back gracefully).

[temcguir]

Similarly, we should add a quick unit test (DebugSettingsNavTypeTest.kt) to verify the URL encoding and decoding steps here. We want to ensure that parseValue(serializeAsValue(debugSettings)) returns the original object and that the resulting string is safe for Compose Navigation routes.