Three small fixes: EventEmitter listeners, the MP4 append queue, Panopto's label

[Nawid3333]

Three independent fixes, one commit each. Each commit applies to main on its own.

1. EventEmitter: an event reaches every listener when one removes itself

emit() walks the listener list with every() while listeners change it. The emitter also walks its context list that way. A once() listener removes itself at the index the walk is on, so the next listener moves into that index and is skipped. A handler that destroys its own context does the same to the next context. VideoAnalyzer, AudioAnalyzer and PreviewFrameExtractor destroy their contexts like this on the player's events.

Both walks now go over a copy, as Node's EventEmitter does. Two consequences:

• A listener added during an emit() is not called for that event. That was already true, because every() stops at the length it started with.
• A listener that another listener removes during an emit() still hears that event. This matches Node.

2. MP4: a SourceBuffer call that throws no longer stops the queue

SourceBufferWrapper runs MP4Player's appends and removals one at a time, each after the previous updateend. There were two problems:

• A remove() that threw (InvalidStateError once the MediaSource closed, for example) still set updating. No update had started, so no updateend came, and every later append and removal stayed queued for good.
• An appendBuffer() that threw (QuotaExceededError) left the operation at the head of the queue. Every later call ran it again and was rejected with its error.

A throw from either call now rejects that operation, drops it and runs the next one. Nothing changes when nothing throws.

3. Locales: translate "Accelerated Panopto"

player_source_accelpanopto exists only in en. The other 15 locales show the English label in the Sources browser's mode menu, the build prints "Missing keys" for each of them, and combined-locales.json lacks the key. Each locale's label now follows its own "Accelerated Vimeo", with the name swapped. This commit also fixes a typo in the German Vimeo label ("Beschleunigtest" → "Beschleunigtes").

Verification

Commits 1 and 2: the real modules from main and from this branch, run in Node. SourceBufferWrapper gets a stand-in SourceBuffer that throws on request.

Case	main	this branch
two once() listeners on one event	only the first is called	both are called
a context destroys itself in its handler, another context after it	the second is skipped	both are called
remove() throws, then an append is queued	the append never settles	the append resolves
append A throws, then B and C are queued	A, B and C are rejected; A is run 3 times	A is rejected; B and C resolve

• Build: npm ci + npm run build: all three targets build, and the build prints no "Missing keys".
• Lint: eslint on the changed files: no problems.
• Merging: no conflict with any of my other open PRs (hls2mp4: define normalizePts, which getVideoStartPts calls #559-srt2webvtt: keep the cues of loosely written SubRip files #570, Firefox: let FastStream follow Firefox VPN for a page's stream #577).
• Not run in Chrome or Firefox. Fixes 1 and 2 have been in my fork's Firefox build for a week.

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