feat: Auto-Improve button in QualificationTab

[VictorGjn]

Context

Dual-agent qualification loop (/api/qualification/auto-improve) created in #137 but no UI trigger.

Changes needed

1. Add "Auto-Improve" button in QualificationTab (next to "Run Tests")
2. SSE event handler for loop events (iteration_start, case_done, corrector_start, patches_applied, loop_done)
3. Progress UI: show iteration count, current score, patches being applied
4. Stop button to abort the loop
5. Results view: score progression chart across iterations

Depends on: #137 merge + mount route