Builds on Darvell's mermaid-rendering work. Three small fixes:

- Drop the fixed `diagramHeight()` formula and `minHeight: proxy.size.height`
  on the unzoomed branch. The image now flows at container-width × intrinsic
  aspect, eliminating the 60-150pt of dead space above and below each diagram.

- Skip the inner `ScrollView` when zoom is 1. Previously the wrapping ScrollView
  consumed wheel events even at the natural fit, so the document scroll would
  stall whenever the cursor passed over a diagram. The zoom > 1 branch still
  wraps in a ScrollView (with a pinned 540pt viewport) so panning works.

- Style picker: replace per-option `Toggle`+manual binding with a `Picker`,
  giving native radio semantics and a proper checkmark on the selected style.

Also swaps `foregroundColor` for `foregroundStyle` on the chrome icons.

Co-Authored-By: Darvell <1046915+darvell@users.noreply.github.com>

Two follow-ups on top of the sizing fix:

- Diagram view: drop the stacked-above chrome row and float the toolbar as
  a translucent capsule in the top-right of the diagram (Preview / Quick
  Look style). The diagram now occupies the full box. The capsule is hover-
  revealed, .thinMaterial background with a hairline border.

- Source view: when toggled to source, render with mdv's normal code-block
  chrome — `mermaid` language label top-left, hover-revealed
  wrap / show-diagram / copy buttons top-right, horizontal-scroll or wrap
  layout for the code. The view now feels like every other fenced block in
  the document instead of a one-off.

Context menu also splits by mode: source shows wrap; diagram shows style
picker and export.

Co-Authored-By: Darvell <1046915+darvell@users.noreply.github.com>