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

package.json

@@ -9,9 +9,12 @@
     "build": "vite build",
     "preview": "vite preview",
     "check": "svelte-check --tsconfig ./jsconfig.json",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "format": "prettier --write ."
   },
   "dependencies": {
+    "@fontsource/be-vietnam-pro": "^5.2.0",
     "@sveltejs/adapter-static": "^3.0.8",
     "@sveltejs/kit": "^2.5.0",
     "@sveltejs/vite-plugin-svelte": "^4.0.0",
@@ -24,6 +27,7 @@
   "devDependencies": {
     "svelte-check": "^4.0.0",
     "prettier": "^3.3.0",
-    "prettier-plugin-svelte": "^3.2.0"
+    "prettier-plugin-svelte": "^3.2.0",
+    "vitest": "^3.2.0"
   }
 }