feat: uniform control height, chrome auto-hide, streaming quality

Every control in the app now shares one height (CONTROL_H), with width
still growing to fit its label. The player transport is larger, Back is
an icon, and the footer buttons match the prev/next size.

Player chrome — transport and edge arrows — fades after 2.6s of
inactivity and never hides while paused.

The title-bar strip collapses in macOS window fullscreen, where the
traffic lights are gone and it was just a blank white bar.

Streaming quality is now selectable alongside download quality. A cap is
applied by rewriting YouTube's HLS master playlist down to the best
variant at or below the chosen height, keeping its audio group. That
playlist is served from a small loopback HTTP server: Safari's native
HLS is backed by AVFoundation, which cannot read blob: or custom-scheme
URLs, so a Blob URL silently fails to play.

Settings closes with an icon button and its selects match the shared
control height.

src-tauri/capabilities/default.json

@@ -9,6 +9,7 @@
     "core:default",
     "opener:default",
     "dialog:default",
-    "core:window:allow-start-dragging"
+    "core:window:allow-start-dragging",
+    "core:window:allow-is-fullscreen"
   ]
 }