fix: contextual logging, word merge overlap resolver, and responsive toolbar/footer UI

Fix focus mode: correctly hide sidebar panels and allow footer to be hoverable to exit focus mode

revert: restore backend/model files to cf07939 state, keep UI/UX changes only

feat: P1-P3 all 8 items - ta marbuta fix, UI feedback, caching, rate limiting, unit tests

fix: Pipeline hardening - 37 bugs + 10 edge cases (Round 1 + Round 2)