feat(a11y): color independence, reduce motion, reduce transparency (#21 phase 5)

Audit unchanged at 11 dark / 14 light — expected, as none of these
settings are exercised by performAccessibilityAudit, and simctl can
toggle only Increase Contrast, not Differentiate Without Color, Reduce
Motion, or Reduce Transparency. Correct by construction and build-clean;
runtime behaviour is verified in the Phase 6 manual pass.

Color independence:

- Widget status is now an SF Symbol (checkmark.circle.fill /
  exclamationmark.triangle.fill / exclamationmark.octagon.fill), the same
  vocabulary as the in-app badges, replacing a silent colour-only dot on
  both the domain rows and the small-view count pills. Status now survives
  greyscale and reads consistently across surfaces.
- Under accessibilityDifferentiateWithoutColor: the Dashboard
  summary-card dot becomes a per-filter symbol, the selected quick-filter
  chip gains a checkmark and a border (selection was fill-colour only, and
  also gains the .isSelected trait), and LabeledValueRow prefixes a
  warning/failure symbol. All gated on the setting so the default UI stays
  uncluttered.

Reduce motion: all five withAnimation/.animation sites now pass nil under
accessibilityReduceMotion — AppCopyButton's check cross-fade,
CollapsibleSectionView's expand/collapse, TimelineDiffView's scroll, and
WatchlistView's list reorder.

Reduce transparency: the single .thinMaterial capsule falls back to an
opaque AppSurfaceElevated fill under accessibilityReduceTransparency.

The SweepActivityController item from the plan is dropped: it is pure
ActivityKit lifecycle with no animation, confirmed back in the issue
triage.

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