feat: port 3 interactive geometry lessons + geom-engine

- Add pure geom-engine module (vec, triangle, circle, ticks) with 34 vitest tests
- Add 3 lessons under /hinh-hoc/: tam-giac-bang-nhau (SSS), tam-giac-dong-dang (similarity), goc-noi-tiep (inscribed angle)
- Add reactive draggable Svelte action with arrow-key a11y
- Add per-lesson colocated i18n + site chrome + lesson registry
- Enable Hình học topic card on landing; keep Số học/Đại số as Sắp ra mắt
- Codify pedagogical tick palette as Tailwind colors.pair.{1,2,3}
- Add Be Vietnam Pro via @fontsource

tailwind.config.js

@@ -4,7 +4,15 @@ export default {
   theme: {
     extend: {
       fontFamily: {
-        sans: ['system-ui', 'sans-serif'],
+        sans: ['"Be Vietnam Pro"', 'system-ui', 'sans-serif'],
+      },
+      colors: {
+        // Pedagogical tick palette — one color per side-correspondence pair.
+        pair: {
+          1: '#D7263D',
+          2: '#1B998B',
+          3: '#F46036',
+        },
       },
     },
   },