[Formulus] feat: Clear local state when switching Synkronus servers

[najuna-brian]

Description

Implements full local data reset when users switch Synkronus server URLs to prevent cross-server data corruption.

Changes

• Added ServerSwitchService to orchestrate complete local state reset
• Updated SettingsScreen to detect server URL changes and prompt user
• Shows unsynced data counts (observations + attachments) before reset
• Offers three options:
  • Sync then switch (recommended): Syncs all data first, then resets
  • Proceed without syncing: Immediately wipes and switches
  • Cancel: Aborts the operation

What Gets Reset

• WatermelonDB (all observations)
• Sync cursors (`@last_seen_version`, `@last_attachment_version`)
• App bundle and form definitions (`/app`, `/forms`)
• Attachment files (`/attachments`, `/attachments/pending_upload`)
• Auth tokens and user session
• All AsyncStorage sync/app metadata

Testing

• ✅ Unit tests added for `ServerSwitchService`
• ✅ All tests passing
• ✅ Tested on device.

Resolves #109

[r0ssing]

Looks great! Let's find a good way to solve the error handling conundrum :-)

[r0ssing]

Looks great!