feat(sse): stream re_derive_brief progress event to loading page

[CodeNinjaSarthak]

Changes

• SessionState.progress_message — new optional field, IPC channel between background task and SSE poller
• _run_refinement() — two intermediate store.set() calls with stage messages; all terminal writes clear the field
• sse.py — plan_progress event emitted while progress_message is set; extracted _event() helper
• plan/page.tsx — writes refine:{sessionId} flag to sessionStorage before navigating to loading page
• loading/page.tsx — reads flag, shows 'REFINING' badge and live progress message during refinement

Tests

• 2 new SSE tests in test_sse_progress.py
• 1 frontend smoke test for loading page module
• 27 SSE tests, 40 manager tests passing, mypy + tsc clean

Closes #104