Move Theme State to URL

[xcillik]

Summary

What was done

1. Theme state moved to URL params (?theme=eosc&mode=dark)
The theme switcher state is now shareable via URL. Precedence: URL param > localStorage > hardcoded default. When you switch themes, the URL updates via router.replace (no full navigation) and localStorage is kept in sync as a cache. Browser back/forward re-syncs the theme from the URL.
Files: src/hooks/use-theme-switcher.ts (URL read/write logic), src/components/ThemeSelector.tsx (Suspense boundary for useSearchParams), src/app/layout.tsx (inline FOUC script extended to read URL params).
2. next-themes removed entirely
After auditing all theme-related files, next-themes was found to be strictly redundant: it only managed the .dark class (which the custom system already handled), used a separate localStorage key ("theme" vs "theme-color-mode"), had no concept of branding (data-theme), and no URL param support. The only sync point was a fragile manual dual-call in ThemeSelector (setDefaultTheme(mode) + setTheme(mode)), and any theme change bypassing that call would silently desync the two systems.
Files deleted: src/components/ThemeProvider.tsx, src/components/ThemeInitializer.tsx (the latter was also redundant with the inline FOUC script and contained an unfixed dataset.Theme capitalization bug). next-themes removed from package.json peerDeps and vite.config.lib.ts externals.
3. Sonner made theme-agnostic
The library's Toaster component was the only library file importing next-themes (to read useTheme().theme for toast styling). It now accepts an optional theme prop instead, so the library no longer forces consumers to install next-themes.
File: lib/components/primitives/sonner.tsx.

Why it was needed

The original codebase had four competing mechanisms managing the .dark class using two different localStorage keys, with a single manual call in ThemeSelector keeping them aligned. Adding URL param support on top of that would have made the desync problem worse — URL-triggered theme changes (shared links, browser navigation) bypass ThemeSelector entirely, so next-themes would go stale while the custom system stayed correct. Removing next-themes eliminated the entire class of sync bugs and simplified the architecture to one system, one localStorage key set, and one FOUC script.

Closes #76