Phase 1 of stylix integration (#1457): extract the Catppuccin palette
into a dedicated, separately-linked stylesheet so a theme swap can
replace just that file without rebuilding the rest of the frontend.
- shared/src/theme.css (new): the `:root` palette, moved out of
base.css (which now holds only the body typography it references).
- shared/package.json: export `./theme.css`.
- dashboard/src/theme.css + agent/src/theme.css (new): one-line
re-exports of @hive/shared/theme.css so each package's esbuild emits
its own standalone `dist/static/theme.css` (palette is NOT inlined
into the page bundles).
- both build.mjs: add theme.css to the CSS build list.
- every page (dashboard index/flow/logs, agent index/stats/screen):
link `theme.css` first, ahead of the page CSS, so the `:root` vars
resolve for everything.
- docs/web-ui/css-vars.md: document the split + the no-rebuild rationale.
Behaviour-neutral — same colours, just relocated. Verified both
`npm run build` outputs: theme.css emits standalone (383b) with the
palette; no `--*` palette defs duplicated into common.css/agent.css.
Phase 2 (nix derivation that swaps theme.css from stylix colours) is a
follow-up; touches nix/frontend.nix, coordinating with damocles.
Part of #1457.
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