Adds five built-in color themes (Creamsicle, Midnight Marmalade, Peach
Fizz, Mint Squeeze, Bubblegum) with instant apply and DB persistence.

Adds yt-dlp video download support via a new media_extractor module and
VideoDownloadModal: automatic yt-dlp detection and install, thumbnail
preview, quality selector, and progress tracking.

Adds settings export/import as JSON.

Bug fixes: resume now re-fetches the full file when ETag/Last-Modified
changes on the server; progress events no longer overwrite terminal
download states; app settings are loaded in a single batched DB query.