Straw phase M-1: tap-thumbnail-to-play + hi-res thumbs + Picture-in-Picture

Cobb's real-use feedback: "playing a video is a button under a very
pixelated thumbnail, no playing in window mode, ... where are all the
features." This is the visible-UX pass.

Thumbnails:
- New util/Thumbnails.kt#bestThumbnail picks the highest-res image from
  NewPipeExtractor's List<Image> by w*h pixel area. Was firstOrNull()
  which is the smallest in the sorted list. Applied across Search,
  Channel video rows, Channel avatar + banner, VideoDetail.

Tap-to-play:
- VideoDetail thumbnail is now wrapped in a clickable Box that fires
  onPlay(). Centered semi-transparent black circle with a white play
  triangle overlay so the affordance is obvious. The standalone "Play"
  button below stays for accessibility (tap target consistency).

Picture-in-Picture:
- Manifest: android:supportsPictureInPicture="true" on StrawActivity +
  added screenLayout|smallestScreenSize|screenSize to configChanges so
  rotation into PiP doesn't recreate.
- PlayerScreen: top-right floating button enters PiP with 16:9 aspect.
- Lifecycle ON_STOP observer now checks isInPictureInPictureMode and
  skips pausing when we're in PiP (was killing playback on PiP entry).

Phase M-2 next: MediaSession + background audio + foreground service
(the "play with screen off / lock screen controls" feature).

strawApp/src/main/AndroidManifest.xml

@@ -16,7 +16,8 @@
             android:name=".StrawActivity"
             android:exported="true"
             android:launchMode="singleTask"
-            android:configChanges="orientation|screenSize|keyboardHidden">
+            android:supportsPictureInPicture="true"
+            android:configChanges="orientation|screenSize|smallestScreenSize|screenLayout|keyboardHidden">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />