refactor(frontend): base16 theme contract in a swappable colors.css

Split the palette into two standalone stylesheets:
- colors.css — the 16 base16 slots (--base00..--base0F). This is the
  entire theme swap contract; a generator (e.g. a stylix base16 scheme,
  which is natively base00-base0F) replaces only this file.
- theme.css — the semantic layer (--bg, --purple, …) derived from the
  base16 slots via var()/color-mix. Never changes on a swap.

Every page links colors.css then theme.css; theme.css does NOT @import
colors.css (that would re-bake the slots into it) — they're separate
dist outputs so a swap touches just colors.css. Pixel-identical refactor:
base16 defaults are Catppuccin Mocha and the three off-slot vars
(--crust, --muted, --subtext0) derive via color-mix reproducing their
exact prior hexes.

Wires colors.css through both build.mjs CSS entry lists, the
@hive/shared exports map, and all 7 page templates. css-vars.md + the
frontend.nix output-list comment updated.

frontend/packages/shared/package.json

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