feat(settings): add remote backend configuration

[slkzgm]

Part of the optional remote backend mode RFC (see #54) and the follow-up work after the daemon prototype landed in #58.

What's included

• Add backendMode (local/remote) + remoteBackendHost (default: 127.0.0.1:4732) + remoteBackendToken to AppSettings.
• Expose these fields in the Settings UI (remote fields are only shown when backendMode = remote).

Notes

• Default behavior remains unchanged (backendMode defaults to local).
• This PR only adds configuration plumbing; it does not switch the app to remote mode yet. A follow-up PR will implement the proxy/event bridging.

Related

• feat(daemon): add workspace management RPC #71 adds daemon-side workspace management RPC (opened in parallel).

Testing

• npm run typecheck
• cargo test (in src-tauri)

[Dimillian]

Deep review feedback:

No issues found in this PR. Settings wiring looks consistent across UI + types + defaults.

Notes/assumptions:

• Remote fields only shown when backendMode=remote; drafts sync looks good.
• Defaults align with backend defaults (127.0.0.1:4732, local).