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/shared/package.json

@@ -8,6 +8,7 @@
   "exports": {
     ".": "./src/index.js",
     "./terminal.js": "./src/terminal.js",
+    "./theme.css": "./src/theme.css",
     "./base.css": "./src/base.css",
     "./terminal.css": "./src/terminal.css"
   },