feat(theme): add Compact Fluent Quad built-in theme preset (#95)

[fix2it]

Summary

Follow-up to #95: adds Compact Fluent Quad as an official built-in theme preset.

Key Features

• 📏 Compact Taskbar Footprint: Calibrated for Windows 11 taskbar proportions alongside the system clock; saves horizontal space by streamlining layout and column margins while preserving high-legibility digits and reset countdowns.
• 🎯 Pixel-Perfect Segment Alignment: Segment bars (9px height) and text baselines are strictly aligned to Segoe UI 12 cap-height and baseline with a balanced 7px rhythm between rows.
• 🎨 Distinct Multi-Provider Palette:
  • Claude Code: Terracotta (#D97757)
  • Codex: Deep Blue (#3B82F6)
  • Gemini / Antigravity: Tech Cyan (#00D2B4)
  • Cursor: Crisp White (#FFFFFF)
• 🔲 Unified 2×2 Quad Tray Icon: Replaces multi-icon clutter with a single clean 2×2 colored matrix tray icon (\shared-tray-icon), while fully retaining hover tooltips and click interactions.

Changes

• Added \src/themes/compact-fluent-quad.json\
• Registered \COMPACT_FLUENT_QUAD_THEME_ID\ in \BUILTIN_THEME_SOURCES\ (\src/theme_engine.rs) so it is automatically available in the managed theme library and context menu.
• Updated \�uilt_in_themes_are_valid_and_cannot_be_saved_as_editable_themes\ in \src/theme_engine/tests.rs\ to validate both built-in themes and ensure zero rendering warnings.

Closes #95.