feat: unify stash and local files

[soundchaser128]

To-dos:

• [x] Fix funscript page (finalFileName in state is really brittle, creating the object URL is also buggy)
• [x] Add button to sync new metadata from Stash for existing videos
• [x] Allow adding newly created markers to Stash as well.
• [x] Changelog entries

Closes #56 Closes #46 Closes #58