feat: durable file-backed persistence for walkie web UI

[jack-arturo]

Summary

Makes the walkie web UI prefer a durable server-side state file (~/.walkie/web-state.json) for identity + joined channels, while keeping localStorage as a resilient offline cache.

This ensures that:

• Name ("jack") and joined channels survive browser clears, profile switches, incognito, and different machines.
• Full service/hub restarts no longer require re-joining channels manually.
• The same state file can be used by the CLI in the future for full parity.

Changes

• Flip load/save priority in the web UI so GET/PUT /state (backed by ~/.walkie/web-state.json) is the primary source.
• Export STATE_FILE, readState, and writeState helpers from src/web.js for future CLI use.
• Fully backward compatible — if no state file exists, behavior is unchanged.

Testing

• Verified manually: wrote state file with 7 channels + name, started server from branch, reloaded browser → UI loaded directly into chat with all channels present (no name prompt).
• Existing web tests still pass (modulo daemon startup flakiness in the test env).

This is a small, focused improvement that builds directly on the recent browser persistence work (#10).

Ready for review. Feedback welcome!