Add a native Slint front-end for the music player

The React UI is sluggish on the kiosk and the reasons are browser-shaped: a compositor
deciding what gets its own layer, a requestAnimationFrame loop repainting the viewport,
and a JPEG decode per album card per cold start. The last three commits chased that
through the service worker and the cover pipeline. This is the other direction - the
same backend, the same layout, the same German copy, without a browser.

Browse and play only. The typing game, the room-lights page, parent mode and the
IR-remote assignment stay web-only, and nothing here touches python-backend.

Four things carry the performance claim, in rough order of how much they should matter:

Covers are downscaled once, ever. The backend serves one size - 640px on the long edge,
no ?size= - and a grid card is 180. src/covers.rs fetches each cover once on a worker
thread, resizes it, and writes a JPEG thumbnail to ~/.cache/musicmouse-slint/covers/.
Two tiers and only two, which is what makes the cache worth keeping: a per-widget pixel
size would give each album a dozen near-identical files and a fresh decode for each. The
cache is dropped when the websocket announces a rescan, because that is the one moment
the backend rewrites the art behind an unchanged cover URL - the trap sw.js had to learn
about the hard way.

The grid is virtualized. Rust hands the UI the album list pre-chunked into rows and the
view puts those in a ListView, which instantiates only what is on screen. A flat list of
660 cards gives it no rows to skip, hence the chunking. Same purpose as
content-visibility: auto on .grid > .card.

The progress bar animates between the 2 Hz pushes rather than running a clock, so
interpolation costs a property evaluation per frame on the render side and there is no
equivalent of usePlaybackClock. It is suppressed for the frame a track changes on, so a
new track jumps instead of sliding across two unrelated positions.

Nothing on screen animates by itself. The ambient canvas is not ported, for the reason
lib/lowPower.ts already gives: a loop repainting the viewport is a floor you cannot get
under while it runs at all.

The device must not use FemtoVG. On Mesa V3D it draws every runtime-loaded Image as
solid black (slint-ui/slint#11785, open), which here means every album cover; Skia and
the software renderer are unaffected. So `kiosk` is linuxkms + Skia and FemtoVG stays
the default only for desktop development. Both profiles are verified to build; the kiosk
binary links libinput/libgbm/libdrm and no X11 or Wayland at all.

lib/search.ts, lib/keyboard.ts and lib/format.ts were already pure functions with their
own tests, so they port across as pure Rust with theirs: 43 tests, no window required.
The .slint files are layout only - nothing in them formats a number or picks a word.

Verified against the real 660-album library rather than a fixture. tools/headless-shots.sh
renders the UI inside a nested headless compositor and grabs a frame per screen, which
is how that was checked on a machine whose session was locked; a Slint window needs a
real compositor, and under bare Xvfb nothing maps at all.

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

slint-frontend/ui/models.slint

@@ -0,0 +1,91 @@
+// The shapes that cross between Rust and the UI.
+//
+// Everything here is already *presentation*: labels are formatted, durations are
+// strings, covers are decoded images. Nothing in the .slint files formats a number or
+// picks a word, so the German copy and the duration format live in one place (Rust)
+// rather than being split across two languages.
+//
+// `nav` is each item's position in the single flat selection order that runs songs ->
+// categories -> albums, so one pair of arrow keys walks the whole page. It is assigned
+// in Rust, where the three lists are built together.
+
+export struct AlbumTile {
+    id: string,
+    title: string,
+    artist: string,
+    // "12 Songs · 45:03 · 🧸" - already assembled, including the figurine marker.
+    meta: string,
+    cover: image,
+    is-book: bool,
+    locked: bool,
+    // This album is the one playing; drawn with the accent ring rather than the
+    // selection ring, and outranked by it when they coincide.
+    current: bool,
+    nav: int,
+}
+
+// The grid is handed to the UI pre-chunked into rows, because a `ListView` virtualizes
+// its model and a flat list of 343 cards has no rows to skip. This is the direct
+// counterpart of the web app's `content-visibility: auto` on `.grid > .card`, which
+// exists for exactly the same measurement: 340 live cards saturated the Pi's renderer.
+export struct GridRow {
+    tiles: [AlbumTile],
+}
+
+export struct CategoryTile {
+    key: string,
+    // "7 Hörbücher"
+    count-label: string,
+    // Up to four covers in a 2x2, or one filling the tile when the category holds a
+    // single album. Four fields rather than an array: the count is fixed and small, and
+    // this keeps the tile a plain struct.
+    cover1: image,
+    cover2: image,
+    cover3: image,
+    cover4: image,
+    cover-count: int,
+    is-book: bool,
+    nav: int,
+}
+
+export struct CategoryRow {
+    tiles: [CategoryTile],
+}
+
+// One of the three shelves on the root screen.
+export struct Shelf {
+    label: string,
+    icon: string,
+    tiles: [CategoryTile],
+    empty-label: string,
+}
+
+export struct TrackRow {
+    title: string,
+    subtitle: string,
+    duration: string,
+    cover: image,
+    is-book: bool,
+    current: bool,
+    locked: bool,
+    album-id: string,
+    index: int,
+    nav: int,
+}
+
+// What the player bar and the full-screen view both say. Shared so the two cannot
+// drift apart on a copy change.
+export struct NowPlaying {
+    title: string,
+    subtitle: string,
+    // "Song 3 von 8" / "Kapitel 3 von 8"
+    counter: string,
+    cover: image,
+    hero-cover: image,
+    is-book: bool,
+    has-album: bool,
+    // "−4:12", the time left in this track.
+    clock-label: string,
+    // "Noch 38:20 im Album" - empty for a podcast, where it means nothing.
+    remaining-label: string,
+}