feat(frontend): split theme CSS vars into a standalone theme.css

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.

frontend/packages/agent/src/theme.css

@@ -0,0 +1,6 @@
+/* Standalone theme bundle — re-exports the shared Catppuccin palette so
+   esbuild emits it as its own `dist/static/theme.css`, linked first by
+   every agent page. Kept separate from the page bundles so a theme swap
+   (e.g. stylix) replaces only this file. See @hive/shared/theme.css +
+   docs/web-ui/css-vars.md. */
+@import "@hive/shared/theme.css";