feat(mini-player): floating mini window — early alpha (#162)

A second webview window (label "mini") with a compact player: album art,
title, artist, prev/play/next, progress bar, pin-on-top toggle, expand
back to main, close. Main minimizes on open and restores when the mini
is hidden or closed. Spacebar toggles, arrow keys skip tracks.

Cross-window sync: main window subscribes to playerStore and pushes
`mini:sync` via emitTo on track / play-state change. Audio progress
already broadcasts to all windows via `audio:progress`. Control actions
(prev/next/toggle) come back via `mini:control`; main-window restore
goes through a direct Rust command because WebKitGTK pauses JS in a
minimized webview.

Tiling-WM detection reused from existing `is_tiling_wm()` — always-on-top
is skipped on Hyprland/Sway/i3 since it's ignored there anyway.

Early alpha: no queue expand, no lyrics, no EQ, no drag-snap. Scope kept
deliberately tight for v1; follow-ups planned.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/capabilities/default.json

@@ -3,7 +3,7 @@
   "identifier": "default",
   "description": "Default capabilities for Psysonic",
   "platforms": ["linux", "macOS", "windows"],
-  "windows": ["main"],
+  "windows": ["main", "mini"],
   "permissions": [
     "core:default",
     "shell:default",