7479ee90

By: Michael Lynch <git@mtlynch.io>

Simplify MediaItem.Filenames() with an extension table and shared variant helper

Now that all resized variants share the same "-<width>px.jpg" naming,
the only per-type difference left in Filenames() is the full-size file
extension. Replace the extension switch with a lookup table and build
all three resized variants (display, desktop thumbnail, mobile
thumbnail) through one helper instead of three hand-written literals.

This adapts the better-filenames branch, whose per-type spec table is
no longer needed after the thumbnail suffix unification.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>