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/src/base.css

@@ -1,25 +1,10 @@
-/* Base palette + typography shared by the hive-c0re dashboard and the
-   hive-ag3nt web UI. Catppuccin Mocha. Per-page stylesheets append on
-   top of this and must NOT redeclare the colour variables — the whole
-   point of pulling them out is one source of truth. */
-:root {
-  --bg: #1e1e2e;        /* base */
-  --bg-elev: #181825;   /* mantle */
-  --crust: #11111b;     /* crust — terminal background */
-  --fg: #cdd6f4;        /* text */
-  --muted: #7f849c;     /* overlay1 */
-  --purple: #cba6f7;    /* mauve */
-  --purple-dim: #45475a;/* surface1 */
-  --cyan: #89dceb;      /* sky */
-  --blue: #89b4fa;      /* blue */
-  --pink: #f5c2e7;      /* pink */
-  --amber: #fab387;     /* peach */
-  --yellow: #f9e2af;    /* yellow */
-  --green: #a6e3a1;     /* green */
-  --red: #f38ba8;       /* red */
-  --border: #313244;    /* surface0 */
-  --subtext0: #a6adc8;  /* subtext0 — secondary/toolbar text, dimmer than --fg but lighter than --muted */
-}
+/* Base typography shared by the hive-c0re dashboard and the hive-ag3nt
+   web UI. The colour variables it references (`--bg`, `--fg`, …) live in
+   the standalone `theme.css` (Catppuccin Mocha), linked separately by
+   every page BEFORE this file so the `:root` vars resolve — see
+   `theme.css` + docs/web-ui/css-vars.md. Per-page stylesheets append on
+   top of this and must NOT redeclare the colour variables; `theme.css`
+   is the one source of truth. */
 body {
   background: var(--bg);
   color: var(--fg);