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-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;