feat: add bubbletint library, theme picker modal, and custom themes support

[Raina-Hardik]

Summary

Integrates github.com/lrstanley/bubbletint/v2 into taskii, providing an interactive fuzzy-search theme browser with 340+ themes, bidirectional curated cycling, adaptive palette conversions, and extensible custom theme loading.

Key Changes

1. Interactive Theme Browser (alt+t):

   • Opens a modal dialog with real-time fuzzy filtering across all 340+ built-in bubbletint themes, curated themes, and user custom themes.
   • ↑/↓ (or ctrl+p/ctrl+n) navigates items with instant live-preview across the entire dashboard.
   • Enter confirms and persists the selected theme to data/settings.json.
   • Esc / ctrl+c cancels and restores the original theme.
   • Distinct source badges: ★ curated (gold/accent), ⚙ tint (muted/accent), and ✦ custom (green/accent).

2. Curated Quick Cycling:

   • t cycles forward through the 7 curated core themes (Ember, Tokyo Night, Dracula, Nord, Light, Monokai, Everforest).
   • T cycles backward through the curated themes.

3. Adaptive Tint Conversion (tintToTheme):

   • Converts standard bubbletint palettes into taskii's 3-layer surface model (Bg < PaneBg < Panel < Border), calculating contrast-aware AppTitleFg and generating a 5-step monotonic HeatmapRamp.

4. Extensible Custom Themes:

   • Discovers custom .json and .jsonl files across cross-platform XDG config/data directories via github.com/adrg/xdg ($XDG_CONFIG_HOME/taskii/themes/, ~/Library/Application Support/taskii/themes/, %AppData%\taskii\themes\) and project-local data/themes/.
   • Supports both full native Taskii Theme objects and standard terminal/Bubbletint Tint objects.

5. Documentation & Tests:

   • Added unit test suite covering registry initialization, cycling, theme resolution, search filtering, custom parsing, and modal layout dimensions.
   • Updated README.md and AGENTS.md with keybindings, directories, and example JSON snippets.

[parsaenami]

Thanks for your contribution to this project!
Also a nice feature! I have a few suggestions on this. I'll leave comments and let you know.

Peace