feat(ui): Analyze/Refine top-bar flow with incremental refinement

[jakob1379]

Summary

Top bar. Two plain buttons replace the in-panel refinement banner.

• Analyze is blue before an analysis exists and red once one is loaded. In the red state a first click arms it ("Confirm re-analyze?") and a second click within 3 s runs it; blur or timeout disarms.
• Refine runs LLM refinement, reads "Refine (update)" when a baseline exists, swaps to "Refining… ✕" while a job runs, and is disabled with a tooltip naming the reason (no analysis / analysis running / no AI provider).

Refinement backend.

• Incremental refinement carries the previous refined grouping onto a fresh analysis and asks the LLM only for adjustments (carry_over_grouping).
• In-flight refinement jobs can be cancelled.
• RefinementResult records head_sha and the covered files; older cached JSON still deserializes (tested).
• After re-analysis a cached refinement is auto-applied only when its head commit and its full file set (grouped + infrastructure) match the fresh diff; otherwise it becomes the baseline for the next incremental update. This fixes File 'src/sepseeq/cli.py' not found in diff after re-analyzing with a stale cached refinement.

Also: PR/MR head polling with a "New commits" bar, and a typecheck:e2e npm script so Playwright spec files are type-checked without a browser.

Review

Three independent reviews were run on the first cut (OpenCodeReview delegation, an over-engineering pass, and a senior code review). Fixed in this PR:

• e2e specs broken by a bad selector rewrite (would have failed to parse).
• Re-analyze during a running refinement: the job is cancelled and its pending stream rejected, so the top bar cannot stick on "Refining…"; a job that completes after a fresh analyze no longer overwrites last_analysis/last_refinement.
• In-memory refinement baseline cleared on every analyze, and the frontend baseline flag reset, so a baseline cannot leak across repos/branches.
• Auto-apply guard now requires full file-set equality including infrastructure files.
• Carried-over groups keep only files the fresh pass still groups, so infra-classified files are not rebuilt with zero-line metadata.
• PR head watcher no longer emits a phantom "new commits" event when its seed poll failed.
• Duplicate incremental-baseline block in start_refine_groups / refine_groups folded into one helper.
• Removed a stray 5 MB output.gif from the commit.

Deliberately not taken: replacing the two-click confirm with window.confirm (design choice), dropping the incremental flag from the command API, and the larger watch_git_head/watch_pr_head and ls-remote dedupe refactors. Those are follow-ups if wanted.

Tests

• cargo test -p diffcore-core -p diffcore-tauri: 2229 passed, 0 failed.
• npx tsc --noEmit and npm run typecheck:e2e: clean.
• npm run build: OK.
• Playwright was not run: its bundled Chromium cannot launch on this machine (missing libglib). The top-bar states were verified manually in the built app instead.