feat(slint): match header dropdown icons, align tab text, tighten menus

Header nav tabs now use the same icons as Tauri (compass, music
library, hard drive, cassette) and the dropdown sub-items carry
leading glyphs everywhere except Discover, which stays text-only.
Tab label text is centered vertically against its glyph and the
header. Dropdown and context menus drop from Radius.md to Radius.sm
so their corners match buttons and album cards.

cc

crates/qbz-slint/ui/assets/icons/folder.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M20 20a2 2 0 0 0 2-2V8a2 2 0 0 0-2-2h-7.9a2 2 0 0 1-1.69-.9L9.6 3.9A2 2 0 0 0 7.93 3H4a2 2 0 0 0-2 2v13a2 2 0 0 0 2 2Z" />
+</svg>