fix: map char level ranges to prosemirror positions

ProseMirror's document model is a tree structure where positions are
calculated by traversing nodes, accounting for structural elements
(opening/closing tags of nodes like paragraphs, lists, etc.). A position
in ProseMirror is not "character index N". Structual boundaries like
"start of paragraph" also get their own index. A document with two
paragraphs "ab" and "cd" has positions:

  0   1 2 3   4   5 6 7   8
  <p> a b </p><p> c d </p>

So when I serialize a ProseMirror document to plain text for external
processing:

   01 234
  "ab\ncd"

I need to save this mapping information so when the external system
says "there's something interesting at characters [3,4]", I can map that
back to ProseMirror positions to do something useful: highlighting,
applying marks, or making selections.

fix: measure site visits

frontend/package.json

@@ -40,10 +40,12 @@
     "globals": "16.4.0",
     "prettier": "3.6.2",
     "tailwindcss": "^4.1.16",
+    "tsx": "^4.21.0",
     "typescript": "5.4.5",
     "typescript-eslint": "8.38.0",
     "typescript-plugin-css-modules": "latest",
     "undici": "*",
+    "uvu": "^0.5.6",
     "vite": "7.1.11",
     "vite-tsconfig-paths": "^4.2.1"
   },