Random video starts and RAW photos

Long videos no longer have to show me the same opening every time they come around, and RAW photos no longer have to be invisible just because the ordinary image surface can’t decode them. Both make the local test rotation feel more like the library I actually want to use.

Changelog

• Videos now honor the Random Spot start setting in Local Samples playback when the clip is long enough to seek safely.
• Beginning mode remains unchanged, and fallback cases are now explicit instead of silently treating every video as beginning-only.
• Local Samples now includes DNG RAW photo exports as photo candidates instead of counting them as unsupported files.
• If a RAW file cannot be decoded by the simulator image surface, the app uses its unsupported local media state instead of crashing.
• Local Samples DNG RAW photos now render through a RAW preview path instead of appearing as black media in the simulator.
• If a RAW preview cannot be decoded visibly, the app shows its local unsupported-media state instead of skipping the item or showing a black frame.