Codify the desktop overlay/design conventions in apps/desktop/DESIGN.md:
surfaces & elevation (shadow-nous + --stroke-nous), stroke/color tokens, the
single Button (variants/sizes, no per-call overrides), shared form controls
(controlVariants / SearchField / SegmentedControl / Switch), flat layout
(PAGE_INSET_X, OverlaySplitLayout, ListRow, no card-in-card), feedback states
(Loader / ErrorState / LogView / EmptyState), BrandMark, motion, i18n, and the
nanostore state model. Ends with a pre-merge checklist.
Two fixes so the doc isn't aspirational:
- brand-mark: rounded-md + overflow-hidden (doc says "softly rounded")
- i18n ja/zh/zh-hant: mirror en's "Begin" + drop trailing period on
connectedProvider (doc says update all locales together)
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