test(tui): add Ratatui app coverage

[alxhall]

This pull request significantly improves the test coverage and reliability of the toki-tui crate by introducing comprehensive unit and integration tests across the application's core modules. It also adds a new test_support module to provide reusable test helpers, and updates the documentation to guide contributors on running the test suite.

Testing and test infrastructure improvements:

• Added a new test_support module (toki-tui/src/test_support.rs) containing helpers for creating test data and initializing the app, projects, activities, time entries, and timers. This module is now included in the crate for use in tests. [1] [2]
• Updated the README.md with a new "Testing" section explaining how to run the TUI test suite and what areas the tests cover.

Expanded test coverage:

• Added a comprehensive suite of unit tests to toki-tui/src/app/mod.rs for core app behaviors, including timer state transitions, project/activity filtering, note editing, and task export parsing.
• Introduced tests for the text input component in toki-tui/src/app/state.rs, covering UTF-8 handling and cursor movement.
• Added tests for runtime actions in toki-tui/src/runtime/actions.rs, including starting/stopping timers, saving/cancelling actions, and deleting entries, with both synchronous and asynchronous scenarios.
• Implemented focused UI rendering assertions in toki-tui/src/ui/mod.rs to verify that key UI states and overlays render as expected, including timer display, project/activity info, status messages, and masked password fields.

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