Loaded video/audio/file blobs stayed in memory for the whole session
(object URLs were only revoked on unmount), so opening several tens-of-MB
videos in one chat grew memory without bound. Explicitly loaded media now
goes through a shared cache that keeps at most three entries; the least
recently used object URL is revoked on overflow and its card falls back to
the Load button (a sticky eviction flag prevents a slow load from
re-publishing an already-revoked URL). Auto-loaded images keep their
existing per-card lifetime.
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