feat: Add experimental rewrite of go2rtc live provider (MSE/WebRTC/MP4/MJPEG) (#2580)

- Closes #2556
- Closes #2450

**Key intended features:**
 - go2rtc compatible
- 100% test coverage to significantly improve ability to test, maintain
and work around browser weirdnesses (e.g. Safari).
 - Written from the ground up in the style of the rest of the project.

**To use:**
 - Change `live_provider` from `go2rtc` to `go2rtc-experimental`.

src/scss/live-go2rtc-experimental.scss

@@ -0,0 +1,33 @@
+@use 'media-layout.scss';
+
+:host {
+  width: 100%;
+  height: 100%;
+  display: block;
+}
+
+video,
+img {
+  @include media-layout.media-layout();
+
+  width: 100%;
+  height: 100%;
+  display: block;
+
+  // The inactive surface is `?hidden` in the template (only the committed one
+  // is shown).
+  &[hidden] {
+    display: none;
+  }
+}
+
+// Hide the native seek bar (and its time readouts) on the live video: the
+// stream is live, so scrubbing back only fights the live-edge chase and snaps
+// to live. Play/pause/volume/fullscreen remain. WebKit/Blink only (Chrome,
+// Safari, Edge, Android); Firefox does not expose these controls for styling,
+// so its scrubber stays.
+video::-webkit-media-controls-timeline,
+video::-webkit-media-controls-current-time-display,
+video::-webkit-media-controls-time-remaining-display {
+  display: none;
+}

src/scss/live-provider.scss

@@ -4,10 +4,26 @@
   position: relative;
 }
 
+// Until loaded media or a snapshot sizes the frame (the `sized` attribute),
+// nothing gives it a size, so reserve a default ratio to stop it collapsing.
+// Real media is never letterboxed into this ratio: the attribute is set the
+// moment it sizes the frame.
+:host(:not([sized])) {
+  aspect-ratio: 16 / 9;
+}
+
 .hidden {
   display: none;
 }
 
+// Fills the frame with a loading or error status when neither media nor a
+// snapshot is present (a provider renders only its own media, so the frame
+// would otherwise be blank).
+.fill {
+  position: absolute;
+  inset: 0;
+}
+
 advanced-camera-card-icon {
   position: absolute;
   top: 10px;