feat: banner gradient toggle that follows the terminal palette (#336)

[LargeModGames]

Closes the follow-up request on #336: the Terminal (ANSI) theme preset recolors the whole UI live with the terminal palette, but the home banner kept its animated RGB gradient, which converts theme colors to hardcoded RGB and therefore never follows palette changes.

What's new

• Banner Gradient toggle (Settings -> Theme -> Banner Gradient, or behavior.banner_gradient in config.yml). When off, the banner is drawn in the theme's banner color as-is, so ANSI colors pass through and tools like pywal recolor the banner live along with the rest of the UI.
• Preset-aware default: the Terminal (ANSI) preset defaults the gradient off (a fixed RGB gradient defeats the preset's purpose); every other preset defaults to on. An explicit banner_gradient: value in the config always wins. Cycling presets in the Settings screen syncs the toggle to the new preset's default, same as the color slots; Custom leaves it untouched.
• No wasted redraws: the Home screen's fast animation tick only ran to animate the gradient, so it is skipped when the banner is static.

Tests

• banner_gradient_toggle_switches_between_rgb_and_theme_color: renders Home both ways and asserts banner glyphs are RGB with the gradient on and exactly theme.banner (ANSI Cyan under the Terminal preset) with it off.
• banner_gradient_defaults_off_for_terminal_preset_unless_explicit: config-load resolution (preset default vs explicit override).
• cycling_preset_to_terminal_turns_banner_gradient_setting_off: Settings preset cycling flips the toggle Default -> Terminal -> Pookie Pink.

Verified manually on Windows Terminal: with the Terminal preset and the gradient off, the banner renders solid in the scheme's cyan and recolors instantly when switching color schemes (Campbell, Solarized Dark, One Half Light), no restart needed.

cargo fmt, clippy -D warnings (telemetry feature set), and the full test suite (439) pass.

Summary by CodeRabbit

• New Features

  • Added a Banner Gradient setting for the home screen.
  • Choose between an animated RGB gradient and a solid theme-based banner color.
  • Theme presets can provide default behavior, while explicit configuration overrides those defaults.
  • The Terminal (ANSI) preset now defaults to a solid banner.

• Documentation

  • Added configuration documentation and changelog details for the new setting.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between fba6d73 and 532751b.

📒 Files selected for processing (7)

• CHANGELOG.md
• docs/configuration.md
• src/core/app.rs
• src/core/user_config.rs
• src/tui/handlers/settings.rs
• src/tui/runner.rs
• src/tui/ui/home.rs

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📝 Walkthrough

Walkthrough

Adds a configurable behavior.banner_gradient setting with preset defaults, YAML persistence, settings UI synchronization, and conditional home-banner rendering. The Terminal preset disables the gradient by default, while explicit configuration values take precedence.

Changes

Banner Gradient Toggle

Layer / File(s)	Summary
Configuration and preset defaults src/core/user_config.rs, docs/configuration.md, CHANGELOG.md	Adds the configurable and resolved banner_gradient fields, preset defaults, explicit-value precedence, YAML persistence, tests, and documentation.
Settings and preset synchronization src/core/app.rs, src/tui/handlers/settings.rs	Adds the Theme settings toggle, applies edits to user configuration, and synchronizes the toggle when previewing or cycling theme presets.
Home banner rendering src/tui/runner.rs, src/tui/ui/home.rs	Gates Home animation ticks with banner_gradient, and renders either animated RGB text or the theme banner color, with coverage tests.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant UserConfig
  participant SettingsHandler
  participant start_ui
  participant draw_home
  UserConfig->>SettingsHandler: resolved banner_gradient
  SettingsHandler->>SettingsHandler: sync preset default or apply toggle
  SettingsHandler->>start_ui: updated user configuration
  start_ui->>draw_home: animation state and banner_gradient
  draw_home->>draw_home: render gradient or theme.banner color

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🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title uses the required feat prefix and accurately summarizes the banner gradient toggle and terminal palette behavior.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch feature/banner-gradient-toggle

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• Commit simplified code in branch feature/banner-gradient-toggle

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