Support deep links for opening sessions directly in Electric Agents desktop & mobile apps

## Summary

Support deep linking so users can open a session in the Electric Agents desktop or mobile app directly via a link (from email, docs, chat, etc). No dependence on a deployed web UI for routing or fallback in this initial implementation.

## Scope

- **Session Link Format:**
- Use a custom protocol scheme: `electric://session/{id}`
- All shared links and documentation should use this format for now.

- **Desktop (Electron):**
- Register the `electric://` protocol handler at install/launch.
- When a link is opened:
- If app is running, bring to foreground and route to the requested session.
- If app is not running, launch & route to the requested session.
- Support deep linking for sessions only (future-proof path for other entity types).

- **Mobile (Expo/React Native):**
- Register as handler for the `electric://` protocol on iOS & Android (intent filters, URL types).
- On link open:
- If user is not signed in or server isn’t configured, show onboarding/setup, then navigate to target session.
- If app is running, navigate directly to session screen.
- Generate and handle only custom protocol links for now (no web fallback).

- **Testing:**
- Validate functionality on all platforms (Windows, macOS, iOS, Android).
- Test with app running, backgrounded, or closed.

## Out of Scope
- No standard web session links or server-side redirects (as no deployed web UI exists yet).
- No fallback to opening a web browser if the app is not installed.

## Acceptance Criteria
- Users can click/tap `electric://session/{id}` links anywhere (browser, email, chat, etc) and the agents app opens to the correct session on desktop or mobile.
- Appropriate routing for onboarding states (setup/sign-in first, then session).
- Documentation updated to guide users to share/consume `electric://` links.

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See thread discussion for additional details and related code context.