Take the service worker off the cover path, and cut cover art to 384px

Search was still stalling for seconds a keystroke. A trace across five keystrokes said
the renderer main thread was blocked for 9.4 s in a single task with *zero* V8 samples
inside it - no JavaScript ran at all. What ran instead, on the worker pool during those
same 9.4 s: ImageDecodeTask 7.6 s, RasterTask 1.7 s, and only 229 ms of actual "Decode
Image". The main thread was waiting on cover bytes, not computing anything.

Two causes, and the first one was mine. The stale-while-revalidate handler added earlier
today made every cover do a Cache Storage read *plus* a network fetch *plus* a cache
write, all serialised through one worker thread. Measured with
Network.setBypassServiceWorker: worst keystroke 5580 ms through the worker against
461 ms without it. So covers no longer go through the worker at all. That costs nothing
here: this worker only registers on localhost or over HTTPS, which on this setup is the
kiosk on the device itself, where the backend is the same machine and a cache lookup is
strictly more work than asking for the file. The shell caching, which is what makes it
installable, stays.

The second is that decode cost goes with pixel count, and 640 px was headroom for a
tablet at devicePixelRatio 2 that nobody had asked for. A browse grid paints a card
132 px wide; the largest any screen asks for is 340. At 384 px, typing "conni" over 343
albums, keydown to painted, three runs:

  before   3735,  270, 5580,  88,  57 ms
  after     873,   75,   17,  24,  26 ms

`shrink_cover` never scales art up, so dropping the limit cannot be applied by
re-reading the cache - only by going back to the original art. _INDEX_VERSION 6 does
that: the index is discarded and every album is scanned again through store_cover.

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

python-backend/musicmouse/library/cache.py

@@ -47,10 +47,15 @@ __all__ = ["Fingerprint", "LibraryCache"]
 #: files are untouched, so otherwise a change to that logic is invisible until somebody
 #: edits their music folder.
 # 5: every album's `cover` now points into this cache, downscaled, including the ones
-# that used to point straight at a `cover.jpg` in the library folder. Bumping this is
-# what re-points them: an index from version 4 is discarded and the next scan rebuilds
-# it, which is cheap by design - see the module docstring.
-_INDEX_VERSION = 5
+#    that used to point straight at a `cover.jpg` in the library folder.
+# 6: MAX_COVER_PX dropped from 640 to 384. This is what applies it. `shrink_cover` never
+#    scales art *up*, so a cover already written at some size is only ever rewritten
+#    smaller - which means a cache holding 640 px files cannot be brought to 384 px by
+#    re-reading them, only by going back to the original art. Discarding the index does
+#    exactly that: every album is scanned again and `store_cover` runs on the real art.
+#
+# Bumping this is cheap by design - see the module docstring.
+_INDEX_VERSION = 6
 
 
 @dataclass(frozen=True, slots=True)
@@ -266,9 +271,19 @@ def _album_from_json(data: dict[str, Any]) -> Album:
 #: cover it painted, to show it in a 185 px card on a Pi with 2 GB of RAM.
 #:
 #: 340 is the largest any of this app's screens asks for (the play view; the browse grid
-#: asks for 180 and the player bar for 56), so 640 still leaves room for a tablet at
-#: devicePixelRatio 2 and cuts the decode by about twenty times.
-MAX_COVER_PX: Final = 640
+#: asks for 180 and the player bar for 56), so this only has to beat 340 to be lossless
+#: where it shows.
+#:
+#: It was 640 first, for headroom on a tablet at devicePixelRatio 2, and that headroom
+#: turned out to be expensive on the device that actually runs this. Measured on
+#: musicdolphin, typing "conni" over a 343-album library, keydown to painted, worst
+#: keystroke of three runs: 3.4-9.1 s at 640 px against 0.2 s at 384 px. Decode cost
+#: goes with pixel count, and a browse grid paints a card 132 px wide.
+#:
+#: A tablet at devicePixelRatio 2 therefore gets a slightly soft cover on the play view
+#: and nowhere else. That is the trade: a visible sharpness margin nobody asked for, for
+#: a search box that answers a keystroke in a frame.
+MAX_COVER_PX: Final = 384
 
 #: Re-encode quality. At these dimensions the difference from 95 is invisible and the
 #: file is a third of the size.