fix: improve Album Art Mode scaling, square sizing, and diagonal resize (#78)

[klay2000]

Summary

Fixes #78 — buttons in Album Art Mode were tiny in larger windows, the mini desktop window didn't always end up square, and the undecorated mini window had no way to resize diagonally.

• Responsive controls: AlbumArtMode.svelte now sizes the close button, prev/play/next icons, and the play button circle with clamp(min, vmin-based, max) instead of fixed pixel sizes (e.g. width="22" on <svg>, 48px .play-btn). They now scale proportionally whether the window is the 320x320 mini mode or a large fullscreen overlay.
• Reliable square window on entry: enterAlbumArtWindow() in albumArtWindow.js previously called setDecorations(false) followed immediately by setSize(320x320), which on some Linux WMs raced against the decoration change and didn't always land on an exact square. Now it lowers minSize/maxSize first (so the resize isn't clamped against the larger normal minimum), applies the 320x320 size, waits a frame for the decoration change to settle, then re-applies the size once more.
• Diagonal resizing: Added four corner data-tauri-resize-handle elements (nw/ne/sw/se) to AlbumArtMode.svelte, each calling the new startResizeDrag() helper (getCurrentWindow().startResizeDragging(direction)). This is required for Tauri v2 undecorated windows, which draw no resize borders themselves. A minSize/maxSize of 220–800px keeps the mini window within a sane range while still allowing the user to resize it (including diagonally) per the issue request — it isn't hard-locked to a perfect square while dragging, since the issue prioritizes allowing diagonal resize.

Notes on testing

• npm run build and npm test pass (49/49 unit tests, no new pure-logic helpers needed for this change).
• I was not able to test the Tauri-specific window behavior (enterAlbumArtWindow/exitAlbumArtWindow square sizing, and the new resize handles via startResizeDragging) — this requires npm run tauri dev/build on a real desktop environment with a window manager, which isn't feasible in this sandboxed worktree. Please verify on your desktop build:
  1. Enter Album Art Mode from the desktop app — confirm the window becomes an undecorated 320x320 square reliably (try it a few times, including after moving/resizing the main window beforehand).
  2. Hover the corners of the mini window and confirm you can drag-resize diagonally from each corner (nw/ne/sw/se).
  3. Exit Album Art Mode — confirm the window restores its previous size/decorations.
• The control-scaling change (clamp/vmin sizing) can be checked in the browser by toggling Album Art Mode and resizing the window — buttons should stay proportionate at both small and large sizes.

Test plan

• User to verify desktop (Tauri) window behavior: square sizing on entry, diagonal resize handles, restore on exit
• User to verify control sizing looks right in browser at small and large window sizes