feat(ui): multi-theme switching, compact card layout, and v2.9.0 release

[spelech]

Summary

• Multi-Theme Support: Introduced 4 themes with zero-latency switching and persistence:
  • Deep Ocean (Dark): Petrol spruce base with cyan and mint accents.
  • Midnight Blue (Dark): Classic obsidian space navy with royal blue and teal highlights.
  • Lavender Haze (Light): Soft lilac-violet canvas with purple and fuchsia accents.
  • Amber Warmth (Light): Sandstone canvas with terracotta orange and warm amber accents.
• Card & Layout Compaction: Reduced card padding, header spacing, and theme card grid footprints for a cleaner UI.
• Unified Typography Variables: Consolidated all font families, weights, and sizes into CSS custom properties.
• Theme Settings UI: Added interactive appearance cards with color swatches, active indicators, and toast feedback.
• Testing & Version Bump: Added theme unit test coverage (89/89 Vitest tests, 277/277 Pytest tests passing) and bumped version to v2.9.0 across docs and configurations.