Fix three issues found in #666

[alansley]

Three follow-ups on fix-resume-busyloop-streaming-fallback, found reviewing #666. Based directly on 47f25e9, so this should merge into that branch cleanly and fold into #666.

1. Player.cpp — the resume clamp lost its unconditional upper bound. The new check only clamps when the dream's metadata frame count is known, but Dream::frames is an uninitialised int that CacheManager leaves at 0 whenever the server payload omits "frames". So a wrapped last_played_frame (the uint32 wrap from the old VAAPI PTS bug) on a dream with no length metadata goes through unclamped and hangs the decoder at startup — the exact failure the deleted 24h@60fps bound was there to prevent. Restored as an unconditional first check, with the metadata bound as the tighter second one. Both now reset to frame 0 rather than frames - 1: metadata can under-report the real length, and resuming to a single frame that immediately skips on is worse than starting the dream over.

2. JSONStorage.cpp — the new try/catch also catches writes. GetOrSetValue is shared by Get and Set, and the Set callbacks assign through as_bool() / as_uint64(), which throw on the same kind mismatch. A hand-edited value of the wrong type makes Set a silent no-op that still returns true and marks the store dirty — and since Settings::Get repairs a failed read by calling Set + Commit, the repair silently fails too. The bad value becomes unfixable from inside the app, and every read of that key warns and rewrites the whole settings file. Writes now replace the mismatched node with one of the right kind and let the callback write into it.

3. PlaylistManager.cpp — the streaming fallback needs a leash. The dream it returns is by construction not on disk and, never having been preflighted for streaming, has no URL either — so the caller resolves one over the network, synchronously, while holding the update mutex. Offline (the usual reason the cache is empty) that fails, the decision is cleared, and the next update repeats it: the busyloop traded for a blocking one, a timeout per cycle. Now skipped entirely in offline mode and rate-limited to one attempt per 10s otherwise, which also bounds the repeated refetch on a single-entry playlist where the "next" dream is necessarily the one that just played.

Builds clean on Linux (CMake/Vulkan). Not runtime-tested against the offline/empty-cache path — that one's from reading.

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