chore: audit and clean up potentially unused addDevice/removeDevice preference endpoints

[coderabbitai]

The addDevice and removeDevice preference endpoints may be dead code now that bulkUpdateDevices is used on web.

Files with TODO comments:

• src/server/api/routers/userPreferences.ts lines 22, 30
• src/server/api/routers/mobile/preferences.ts lines 28, 36

Action required:

• Check all web and mobile clients to confirm whether addDevice / removeDevice are called anywhere.
• If unused, deprecate and remove them.
• If still used (e.g., mobile), document the reason they are kept.

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Requested by @Producdevity — related to PR #375