fix(player): scale video to fill the player viewport

The <video> element used `max-w-full max-h-full`, which only ever shrinks
oversized media. A source smaller than the window (480p on a 1080p display)
rendered at its intrinsic size — a small picture floating in a black frame.

Fill the container and let `object-contain` do the scaling, so the picture
fits whichever axis constrains it in both directions while preserving aspect
ratio. The sizing rules move to `videoFit.ts` so they are unit-testable
outside the component.

src/lib/components/player/VideoPlayer.svelte

@@ -12,6 +12,7 @@
   import SleepTimerModal from "./SleepTimerModal.svelte";
   import SleepTimerIndicator from "./SleepTimerIndicator.svelte";
   import CachedImage from "../common/CachedImage.svelte";
+  import { videoFitClass } from "./videoFit";
   import { sleepTimerActive, sleepTimerExpiredSignal } from "$lib/stores/sleepTimer";
   import { playbackPosition } from "$lib/stores/player";
   import * as html5Adapter from "$lib/player/html5Adapter";
@@ -1595,7 +1596,7 @@
       <video
           bind:this={videoElement}
           src={currentStreamUrl.includes('.m3u8') && Hls.isSupported() ? '' : currentStreamUrl}
-          class="max-w-full max-h-full"
+          class={videoFitClass()}
           class:invisible={!isMediaReady}
           style="filter: brightness({brightness})"
           playsinline