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

src/app.css

@@ -1,3 +1,7 @@
+@import '@fontsource/be-vietnam-pro/400.css';
+@import '@fontsource/be-vietnam-pro/500.css';
+@import '@fontsource/be-vietnam-pro/700.css';
+
 @tailwind base;
 @tailwind components;
 @tailwind utilities;
@@ -11,5 +15,12 @@
 
   html {
     -webkit-text-size-adjust: 100%;
+    font-family: 'Be Vietnam Pro', system-ui, sans-serif;
+  }
+
+  /* Visible focus ring for keyboard users on draggable SVG vertices. */
+  svg [role='slider']:focus-visible {
+    stroke: #4f46e5;
+    stroke-width: 3;
   }
 }