feat(chat): message fork, edit and flowchat

[Fodesu]

Summary

Adds copy, fork, and request rewrite actions for chat messages, backed by session branch/turn history.

Changes

• Add message copy behavior for user and assistant messages.
• Add assistant message fork action and request rewrite flow.
• Introduce branch/turn persistence for chat history.
• Add branch graph APIs and active branch switching.
• Update message listing/context loading to use active branch visible history.
• Render branch history sidebar from backend branch graph data.
• Preserve branch/turn context for deferred tool approvals and user input continuations.

Tests

• Added backend tests for branch fork/rewrite semantics, visible history, migrations, and continuation context.
• Added frontend tests for copy/fork/rewrite action behavior and branch store flows.
• Verified SQLite/PostgreSQL query generation with sqlc.