feat(a11y): Dynamic Type reflow and tap targets (#21 phase 3)

Takes the audit from 18 findings to 11 in dark mode. Everything that
remains is system-rendered or placeholder noise, characterised below.

The largest win was not where the plan expected. Every empty-state
heading reported as clipped text, and the cause was `Label`: it
constrains its own title, and `.fixedSize` applied to the Label does not
reach the `Text` inside. Splitting into `HStack { Image; Text }` and
putting the modifier on the Text cleared all four empty states at both
default and accessibility sizes.

That fix then caused a regression the audit caught immediately. `Label`
folds its image into the title's accessibility element; an HStack does
not, so the icon began announcing its raw SF Symbol name
("checklist.unchecked") to VoiceOver. Decorative icons split out of a
Label now carry .accessibilityHidden(true).

Tap targets:

- AppCopyButton was a literal 30x30 on nearly every data row. Now
  @ScaledMetric from 44, floored at AppLayout.minimumTapTarget —
  @ScaledMetric scales down below the default text size as well as up,
  so the floor is load-bearing.
- controlMinHeight was 42 in compact density, putting every collapsible
  section header and both Run buttons under the minimum.

Reflow:

- CardView's allowsHorizontalScroll defaulted to true, so nine call sites
  hid content behind a horizontal gesture instead of wrapping — a WCAG
  1.4.10 failure and the mechanism behind clipped rows at large text
  sizes. The default is now false, and the remaining opt-in is suppressed
  at accessibility sizes.
- Fixed .system(size:) point sizes replaced with text styles in the app
  and the widget.
- The widget is clamped at accessibility1, the one place clamping is
  correct: a widget canvas is a fixed size and WidgetKit truncates
  overflow with no scroll affordance.

Two hypotheses were tested and discarded rather than left in. Monospaced
fonts looked like the clipping culprit — the app is 82% monospaced and
hyphenates mid-word at accessibility sizes — but switching the empty
state to proportional changed nothing, and prose typography is a design
decision rather than an accessibility fix. Shortening search prompts and
the domain placeholder also changed nothing: placeholder text is reported
clipped regardless of length, so "Search" is flagged exactly as "Search
portfolio" was.

Not done: ViewThatFits reflow for BatchResultRowView and
WatchlistRowView. Those rows never render in the audit because the test
simulator has no tracked domains or batch results, so any change there
would be unverifiable. Absence of findings is absence of data.

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