Swap the local ThemeProvider for the canonical shape merged in shadcn-ui/ui#10396 and drop the redundant next-themes dep.

The upstream docs ship a provider that uses ScriptOnce from @tanstack/react-router to inject the FOUC-guard script before hydration, a mount flag to defer effects past SSR, and a useTheme that throws when used outside the provider. Our version is now a byte-for-byte match apart from one return undefined to satisfy oxlint's consistent-return rule and oxfmt-driven whitespace.

Toaster previously pulled useTheme from next-themes, which was the only consumer of that package. It now reads from the local provider, so next-themes is gone from package.json.

Separately, .oxfmtrc.json now excludes convex/_generated/** so bun run fmt stops fighting Convex codegen (the generator rewrites semicolons on every convex dev).

• src/components/theme-provider.tsx: rewrite to match shadcn-ui/ui#10396 (mount flag, getThemeScript, applyTheme(theme), throwing useTheme)
• src/components/ui/sonner.tsx: swap next-themes for @/components/theme-provider, drop "use client"
• package.json, bun.lock: remove next-themes
• .oxfmtrc.json: add convex/_generated/** to ignorePatterns