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/dashboard/build.mjs

@@ -86,11 +86,12 @@ await build({
 });
 
 // Bundle CSS — one entry per page. esbuild resolves @import including
-// the package re-exports from @hive/shared. Each page loads theme.css
-// (the standalone Catppuccin palette — kept its own file so a theme
-// swap replaces only it) + common.css (shared typography, badges,
-// buttons, inbox, side panel) plus its own page-specific bundle.
-for (const entry of ['theme.css', 'common.css', 'dashboard.css', 'flow.css', 'logs.css', 'home.css']) {
+// the package re-exports from @hive/shared. Each page loads colors.css
+// (the standalone base16 palette — the theme swap contract, its own file
+// so a swap replaces only it) + theme.css (the semantic derivation
+// layer) + common.css (shared typography, badges, buttons, inbox, side
+// panel) plus its own page-specific bundle.
+for (const entry of ['colors.css', 'theme.css', 'common.css', 'dashboard.css', 'flow.css', 'logs.css', 'home.css']) {
   await build({
     entryPoints: [src(entry)],
     outfile: staticDir(entry),