feat: custom wallpaper upload

[KenobiNakamoto]

Summary

Add support for uploading custom wallpapers to umbrelOS, allowing users to personalise their home screen beyond the built-in options.

Closes #1622. Closes #2077.

What changed

Backend (umbreld)

• New saveCustomWallpaper method on the User class that persists uploaded images to <dataDirectory>/data/wallpapers/<uuid>.jpg and stores a custom:<uuid> reference in the user store.
• New uploadWallpaper tRPC mutation that accepts a base64-encoded JPEG (capped at ~10 MB decoded), validates the JPEG magic bytes, and delegates to saveCustomWallpaper.
• New Express route GET /api/wallpapers/custom/:id to serve custom wallpaper files, placed before the static catch-all.

Frontend (ui)

• resolveWallpaperAssets helper that transparently handles both built-in numeric IDs and custom:<uuid> IDs, so every existing wallpaper lookup works without special-casing.
• Upload button added to the wallpaper picker strip. The browser canvas API is used to convert any input format (PNG, WebP, HEIC, etc.) to JPEG at 0.92 quality before upload — this normalises the format and strips EXIF metadata client-side.
• WallpaperProvider context widened to carry uploadWallpaper and accept the broader WallpaperBase type.

Security considerations

• Path traversal prevention — The custom wallpaper serving route validates :id against a strict UUID v4 regex before constructing a file path. Invalid IDs receive a 400 response; no user input is ever interpolated into a path unchecked.
• File type validation — The backend checks JPEG magic bytes (FF D8 FF) on the decoded buffer. The frontend additionally re-encodes through <canvas> → toBlob('image/jpeg'), so the server only ever receives valid JPEG data.
• Size limit — The base64 payload is capped at 14 MB (≈10 MB decoded), preventing oversized uploads from consuming disk or memory.
• EXIF stripping — Re-encoding via the canvas API discards all EXIF metadata (GPS coordinates, camera info, etc.) before the image leaves the browser.

Out of scope

The following are intentionally left for follow-up work:

• Multiple saved custom wallpapers / a gallery UI
• Server-side thumbnail generation (custom wallpapers reuse the full image as their thumbnail)
• Wallpaper cropping or editing UI
• Non-JPEG format storage

Testing checklist

• Upload a JPEG — wallpaper applies immediately
• Upload a PNG, WebP, or HEIC — converted to JPEG client-side, uploads successfully
• Upload a file larger than 10 MB — rejected gracefully
• Upload a non-image file (rename a .txt to .jpg) — rejected by magic-byte validation
• Switch between custom and built-in wallpapers — transitions work correctly
• Refresh the page after setting a custom wallpaper — wallpaper persists
• Attempt path traversal via GET /api/wallpapers/custom/../../etc/passwd — returns 400
• Verify no EXIF/GPS data is present in the stored file on disk