feat(remote): proxy core commands via daemon

[slkzgm]

Part of the optional remote backend mode RFC (#54).

What

Adds a Tauri-side remote backend proxy layer that can (optionally) forward a subset of backend commands to the daemon introduced in #58.

• New module: src-tauri/src/remote_backend.rs
• Wires the proxy into existing command paths so local mode remains the default and unchanged.

Why

This is the next incremental step after:

• feat(settings): add remote backend configuration #70 (app settings: backendMode, remoteBackendHost, remoteBackendToken)
• feat(daemon): add workspace management RPC #71 (daemon workspace management RPC)

The goal is to allow the UI to keep using the same Tauri commands/events, while the Rust side can swap between:

• local (current behaviour)
• remote (daemon over localhost JSON-RPC)

Notes

• Strictly opt-in behind backendMode=remote.
• No changes to default behaviour.

Validation

• npm ci
• npm run typecheck
• npm run build

Rust compilation/tests require platform WebKitGTK deps on Linux; CI should cover this.