fix(theme): primary button wears the accent in dark mode

[deepiq2022]

Summary

• Fixes a dark-mode defect the test suite couldn't see. The primary button rendered as a pale peach with dark text — Material derives a light primary for dark schemes, so the app's most important control was the one element not wearing Lamplight Terracotta, a flat break of DESIGN.md's Rubrication Rule. Light mode hid it because the amber seed's light primary lands near the token by luck.
• The fill now comes from JuiceTokens at the theme in both brightnesses, with the label flipping per mode: near-white ink on the deep light clay, page-dark ink on the lifted dark clay.
• Ships the primary lift — the second and last shadow DESIGN.md sanctions, and the only one it had left unimplemented. Rides Material elevation with a terracotta shadowColor rather than a literal BoxShadow, so the button lifts without spending the One Shadow Rule's budget and no call site hand-rolls it.
• DESIGN.md and the sidecar updated to match: the Elevation section moves the primary lift from "specified, not yet shipped" to shipped, and the Buttons entry records why the fill is themed rather than derived.

Test plan

• flutter analyze clean (one pre-existing info in test/embark_sheet_ui_test.dart, untouched here)
• flutter test passes — 2192 tests
• Browser-verified at both brightnesses: light is deep terracotta with near-white label and a warm lift; dark is #D0814F with page-dark label
• Campaign identity spines from docs(design): codify DESIGN.md as enforced visual authority #320 confirmed distinct in light and dark, Umber separating from Terracotta by value
• Result card border and sketch toolbar in dark mode — not reachable in this pass (clicks don't land on the Flutter web canvas); both are single-token swaps covered by the guard test

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