refactor(tokens): make token layer authoritative and wire to layout

- Imported tokens.css directly into BaseLayout.astro.
- Removed legacy :root variable definitions from chapters.css, skills-review/styles.css, rules/styles.css, and styles.css.
- Added self-hosted --font-sans and --font-mono to tokens.css and updated legacy font stacks.
- Removed base.css.
- Added a build-output check in check-tokens.mjs to ensure the token layer is loaded in dist html files.

src/styles/base.css

@@ -1,2 +0,0 @@
-/* Base styles and variables imported */
-@import url('./tokens.css');

src/styles/tokens.css

@@ -11,6 +11,10 @@
   --red: #a7483f;
   --violet: #6b668f;
 
+  /* Fonts */
+  --font-sans: manrope, arial, sans-serif;
+  --font-mono: 'DM Mono', monospace;
+
   /* Typography Scale */
   --step-display: clamp(56px, 9vw, 126px);
   --step-6: clamp(36px, 5vw, 65px);