feat(tokens): define global design tokens and typography scale

- Created `src/styles/tokens.css` establishing the canonical 10-color palette, 5 designated breakpoints, and the 6-step typography scale.
- Created `src/styles/base.css` to import the root variables for Astro components.
- Wrote down decisions regarding the `--blue` palette canonicalization and the font stack simplification in the task brief.
- These tokens will power the Astro component refactors in parallel tasks.

src/styles/base.css

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+/* Base styles and variables imported */
+@import url('./tokens.css');

src/styles/tokens.css

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+:root {
+  /* Palette */
+  --accent: #7c78a8;
+  --blue: #527f9f;
+  --deep: #102536;
+  --gold: #efc76b;
+  --ink: #172f42;
+  --line: #d8dee2;
+  --muted: #697b89;
+  --paper: #f5f4f1;
+  --red: #a7483f;
+  --violet: #6b668f;
+
+  /* Typography Scale */
+  --step-display: clamp(56px, 9vw, 126px);
+  --step-6: clamp(36px, 5vw, 65px);
+  --step-5: clamp(24px, 3vw, 38px);
+  --step-4: clamp(22px, 3vw, 36px);
+  --step-1: 15px;
+  --step-0: 11px;
+
+  /* Breakpoints */
+  --bp-sm: 560px;
+  --bp-md: 800px;
+  --bp-lg: 1100px;
+  --bp-xl: 1600px;
+  --bp-2xl: 2200px;
+}