feat(sync): write state under the Stashden names and carry the old ones over

[BryanFRD]

Refs #245. Bottom of the six-PR rename stack.

• A synced folder now keeps .stashden-sync.json, and partial downloads end in .stashpart. Opening a folder that only has .roxycloud-sync.json renames it in place first (adopt_legacy), so a folder an older version synced keeps its history instead of resyncing from scratch. When both exist, the current one wins.
• The scan and the watcher still ignore .roxycloud-sync.json, .roxycloud-sync.tmp and .roxypart, so a file an older version left behind is never uploaded as a user file.
• In the browser and the desktop WebView, every roxycloud.* storage key moves to stashden.* once at startup: the token, the server, and each account's sync folder (roxycloud.sync-folder:<email>). A value already under the new name is not overwritten.

Tested: two engine tests, one per case (taking over the old state keeps a local deletion recognised; with both files the current one wins), with removing the takeover failing the first; watcher tests for the old names. The key migration was checked in a real browser: old token, server and per-account folder moved, an existing new value kept, unrelated keys untouched.

[ferrfleet]

Nothing blocking. The legacy state file is adopted before load and skipped by both the scan and the watcher. The localStorage migration runs in the PLATFORM factory, and sync-view injects that before it reads stashden.sync-folder:*, so the prefixed folder keys carry over as well.