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/components/Sidebar.tsx

@@ -1,5 +1,5 @@
 import type { ChannelWithCount } from "../types";
-import { BTN_CHROME, HEADING } from "./ui";
+import { HEADING, ICON_BTN } from "./ui";
 
 interface Props {
   channels: ChannelWithCount[];
@@ -49,7 +49,7 @@ export default function Sidebar({
             onClick={onOpenSettings}
             title="Settings"
             aria-label="Settings"
-            className={`${BTN_CHROME} grid size-7 cursor-pointer place-items-center`}
+            className={`${ICON_BTN} text-slate-500 hover:bg-slate-100 hover:text-slate-900 dark:text-slate-400 dark:hover:bg-slate-800 dark:hover:text-white`}
           >
             <svg viewBox="0 0 24 24" className="size-4" fill="none" stroke="currentColor" strokeWidth="1.8">
               <circle cx="12" cy="12" r="3.2" />
@@ -60,7 +60,7 @@ export default function Sidebar({
             onClick={onHide}
             title="Hide subscriptions"
             aria-label="Hide subscriptions"
-            className={`${BTN_CHROME} grid size-7 cursor-pointer place-items-center`}
+            className={`${ICON_BTN} text-slate-500 hover:bg-slate-100 hover:text-slate-900 dark:text-slate-400 dark:hover:bg-slate-800 dark:hover:text-white`}
           >
             <svg viewBox="0 0 24 24" className="size-4" fill="none" stroke="currentColor" strokeWidth="1.8">
               <path strokeLinecap="round" strokeLinejoin="round" d="M15 6l-6 6 6 6" />