web: the dashboard page, slice 3 of M3

The milestone 2 parity page becomes the spec's dashboard: app bar with
wordmark, disclaimer badge and About popover, theme toggle (persisted,
defaults to prefers-color-scheme, applied pre-mount to avoid a flash),
hero with the preset narrative and live toned percentages, scenario
toolbar, panel cards with skeleton loading and dim-while-refreshing,
legend, and footer. ComparisonTable evolves into the collapsed
ResultsTable (test moved along, plus a formatPct rounding case).

NetworkView renders the real topology with the spec's shape encodings.
The layout is seeded d3-force (mulberry32 random source, seeded initial
positions, 300 synchronous ticks, cached per graph hash); anisotropic
forceX/forceY pulls settle the cloud into the wide card shape so the
fit stays uniform-scale and the spacing organic. Justin asked for a
taller network area (380x230) and a reading caption under the legend;
the caption is study copy, so StudyPreset gains a readingCaption field.

Slices still to come keep their controls visibly parked: Export and the
panel kebab render disabled until slices 11 and 7.

web/.gitignore

@@ -37,3 +37,4 @@ __screenshots__/
 
 # Vite
 *.timestamp-*-*.mjs
+public/__*.html