feat: initial Peekache implementation

[tomasmach]

Summary

Full V1 implementation of Peekache — a Tauri desktop app for browsing, previewing, and managing cached media from Discord (and future data sources).

Backend (Rust):

• DataSource trait with provider registry for extensible source support
• Discord cache provider with auto-detection per OS
• Custom directory provider for arbitrary paths
• SQLite persistence with filtered/sorted/paginated queries
• Cache engine with scan orchestration, thumbnail generation (image crate + ffmpeg sidecar), and file operations
• 13 Tauri commands covering scan, browse, preview, save, export, delete, clipboard, and settings
• 26 passing tests

Frontend (React + TypeScript + Tailwind):

• Gallery with responsive thumbnail grid, infinite scroll, media type badges
• Filter bar with type chips, source dropdown, sort options
• Preview modal with full-size media, file info, keyboard navigation
• Actions: Save, Export, Delete, Copy to Clipboard, Open in System Viewer, Show Original Location
• Settings panel: source management, save directory, theme toggle, clear thumbnails, about
• Scan summary toast notifications
• Light/dark/system theme with CSS custom properties

Test plan

• Run cargo test — 26 tests should pass
• Run npx tsc --noEmit — 0 TypeScript errors
• Run npm run tauri dev — app should launch
• Verify Discord cache auto-detection on macOS/Linux/Windows
• Scan and browse cached media in the gallery
• Click thumbnails to preview, test Save/Export/Delete/Copy actions
• Test filter chips and source dropdown
• Toggle light/dark/system themes
• Add/remove custom directory in Settings