fix(web): remediate GUI Crucible findings (a11y, honesty, states, perf)

[jungdaesuh]

Summary

Remediates the confirmed GUI findings from an adversarial (Crucible) review —
accessibility, honesty of the live/mock signal, missing UI states, and a re-render
perf issue. Adopts the existing theme.css token system; minimal token-driven diffs.

Related issue

Relates to #43 — the equilibrium overlay control is disabled/"(coming soon)" until
its backend node lands; the contract branch is intentionally retained.

Type of change

• Analysis core (src/magnetics/core)
• Data source (src/magnetics/data)
• Service / API (src/magnetics/service)
• Web GUI (gui/web)
• Docs
• Other

What changed

• Perf: per-field useStore selectors so the shell no longer re-renders on every
  time-cursor scrub (App didn't read cursorMs).
• Honesty: the "live backend" badge keys off the backend's per-machine mock flag
  (was !!API_BASE, which reads "live" over a mock machine list); RotatingTab's
  data-source label follows suit. Adds mock?: boolean to MachineInfo.
• UI states: SensorsTab empty state for incomplete geometry meta; RotatingTab
  "computing mode map…" placeholder + persistent scrubber while the heavy mode_number
  STFT loads; ErrorBoundary retry; PullControl shot validation (+ inputMode/
  aria-invalid); equilibrium control disabled + "(coming soon)" (Add real equilibrium (LCFS / flux-surface) plotting to the Sensors view #43).
• Accessibility: keyboard-operable tabs / shot list / resize divider / collapse
  headers; aria-pressed toggles; slider labels; role=status/aria-live +
  role=progressbar on the live-pull progress.
• Docs/tests: qsTransforms.phiPeak docstring corrected (signed argmax is the
  intended, physically-correct behavior) + a signed-vs-|·| test; ErrorBoundary retry
  test; cross-tab theme-sync store listener + tests.

How I tested it

In gui/web: npm run lint / npm run typecheck / npm test (28 passed) /
npm run build → all green. (The >500 kB chunk warning is pre-existing/plotly.)

Checklist

• CI is green (analysis: ruff + pytest; web: lint + build) — validated locally
• No secrets/credentials committed
• No large files or tokamak data committed
• Physics kept in core (device-agnostic); none added to service routes
• Docs updated if behavior/usage changed (n/a — no user-facing usage change)

🤖 Generated with Claude Code