fix: v1.1 QA round

[jdpigeon]

Five fixes from the v1.1 manual QA, before tagging.

1. Home: the returning-user subheading "Pick up where you left off…" is gone and the spacing is tighter, so Continue your work starts at y≈183 of 720. The first-run intro sentence stays.
2. Signal quality thresholds raised 50%: noisy ≥22.5 µV, settling ≥15 µV, no-contact floor unchanged. One enum, SIGNAL_QUALITY_THRESHOLDS, shared by the head diagram, the trace colors and Explore's blink calibration.
3. Clean: clicking a suggestion's "Trial N", Accept or Restore pages the reviewer to that trial and outlines it in the gold accent (EpochReviewer focus prop; the focus clears when the arrays change after a save). Storybook check: clicking Trial 27 moved the page from 0–7 to 24–31, outlined it, and caused no page scroll.
4. Clean, pixelated traces: when samples outnumbered pixels the reviewer drew each column's min→max as a separate vertical tick. Now it's one connected min/max path, bucketed per device pixel.
5. Analyze, ERPs across the scalp: MNE's plot_evoked_topo draws no head, so plot_topo now adds a head outline (circle, nose, ears) under the traces. The legend uses explicit handles, which fixes the old swatch mismatch (the TODO is removed). Rendered with native MNE before and after.

Checks:

• npm run typecheck clean.
• vitest for Clean/Home/Explore/eeg: 94 passed.
• pytest tests/analysis: 20 passed.

Not verified here: the thresholds against a real Muse; that needs your re-check.