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/theme.css

@@ -0,0 +1,30 @@
+/* Theme colour variables (Catppuccin Mocha) — the single source of
+   truth for the hive UI palette. Kept in a standalone file, linked as
+   its own `<link rel="stylesheet" href=".../theme.css">` by every page
+   BEFORE the page CSS, so the `:root` vars are defined for everything
+   that references them (base typography, terminal, dashboard, agent).
+
+   Why standalone rather than `@import`ed into the page bundles: esbuild
+   inlines `@import` at build time, which would bake the palette into
+   every CSS bundle. Emitting `theme.css` as its own output file lets a
+   theme swap (e.g. a stylix-generated variant carrying the same var
+   names) replace just this one file without rebuilding the rest of the
+   frontend. See docs/web-ui/css-vars.md. */
+: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 */
+}