tui: make theme configurable via CLI settings

[adityavkk]

Problem

The TUI theme is hardcoded to "openhands" in textual_app.py. There is no way for users to customize the color theme, even though Textual ships with many built-in themes (dracula, nord, catppuccin-mocha, tokyo-night, etc.).

Solution

Add a theme field to CliSettings (persisted in ~/.openhands/cli_config.json) so users can set their preferred Textual theme. The default remains "openhands" for full backward compatibility.

Changes

1. openhands_cli/stores/cli_settings.py: Added theme: str = "openhands" field with a field_validator that checks the value against Textual's BUILTIN_THEMES plus "openhands". Exported VALID_THEMES set.

2. openhands_cli/tui/textual_app.py: Changed self.theme = "openhands" to self.theme = cli_settings.theme, reading from the already-loaded CliSettings. The custom OPENHANDS_THEME is still always registered.

3. openhands_cli/tui/modals/settings/components/cli_settings_tab.py: Added a Select widget for theme selection in the CLI3. openhands_cli/tui/modals/settings/components/cli_settings_tab.py: Added a Select widget for theme selection in the CLI3. openhands_cli/tui/modals/settings/components/cli_settings_tab.py: Added a Select widging3. **open_settings.py: Added tests for theme default value, valid/invalid theme validation, VALID_THEMES contents, save/load roundtrip, and updated the JSON format assertion.

   • tests/tui/modals/settings/t - testttings_tab.py: Added tests for theme selector rendering (default and custom values), get_updated_fields()` reflecting theme changes, and updated the managed-fields assertion.

Verification

make lint   # All checks passed
make test   # 1282 passed, 5 warnings (pre-existing)

Note: Snapshot tests (make test-snapshots) will need updating since the CLI settings tab now includes the new theme selector widget. Not run here as they require a specific environment.

PR Checklist

• Scope is minimal and focused on one change
• Tests added/updated for behavior changes
• make lint
• make test
• (TUI touched) make test-snapshots — snapshots need updating for the new control
• PR description includes: what changed, why, commands run