Fix picker highlight text contrast

[alejandro-ao]

Description

Fix picker selection styling so focused labels use each active theme's configured highlight foreground and background colors.

This corrects the Textual highlight class selector from .--highlight to .-highlight and adds sufficiently specific rules for picker labels whose normal text-color selectors would otherwise win the cascade. The fix covers the theme, model, login-method, and login-provider pickers. A parameterized regression test verifies the theme picker against all three built-in themes.

User experience

Focused theme names are now legible in dark mode: Tau renders dark text over the light aqua selection background instead of white text with insufficient contrast. Picker selections also consistently follow the shared theme palette across built-in themes.

Closes #401.

Manual validation

1. Start Tau's interactive TUI with the tau-dark theme.
2. Run /theme.
3. Move the focused row with the Up and Down arrow keys.
4. Confirm the focused theme name is dark and readable on the light aqua background.
5. Select tau-light and high-contrast, reopen /theme, and confirm each focused row uses that theme's configured selection colors.
6. Optionally open the model and login pickers and verify their focused labels remain readable too.

Checks

• uv run pytest
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• uv build
• hugo --minify (from website/)