docs: add frontend modularization design

Split app.js into ES modules with JSDoc types, no build step.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

docs/plans/2026-02-17-frontend-modularization-design.md

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+# Frontend Modularization Design
+
+## Problem
+
+The frontend is a single 1,436-line `app.js` file handling state, canvas rendering, zoom/pan, SSE streaming, solver control, associate matching, and UI interactions. As the project grows and attracts contributors, this monolith becomes hard to navigate, refactor, and reason about — both for humans and code agents.
+
+## Approach
+
+Split `app.js` into ES modules with JSDoc type annotations. No build step — browsers natively support `<script type="module">`. VS Code enforces types via `// @ts-check` + `jsconfig.json`.
+
+### Why not TypeScript?
+
+- Introduces a build step (`tsc`) and `node_modules` dependency
+- The zero-build simplicity of FastAPI serving static files directly is valuable
+- JSDoc + `@ts-check` gives ~90% of TypeScript's benefits (autocomplete, type errors, hover docs) with zero tooling
+- If TS is desired later, the migration from JSDoc-typed ES modules to `.ts` files is mechanical
+
+## Module Structure
+
+| File | ~Lines | Responsibility |
+|------|--------|----------------|
+| `types.js` | 30 | JSDoc `@typedef` definitions — the data contract layer |
+| `state.js` | 30 | Global `state` object + `getPageRedactions()` |
+| `dom.js` | 65 | All `getElementById` refs + `showToast()` + `escapeHtml()` |
+| `canvas.js` | 250 | `renderCanvas()` + all `drawRedaction*()` functions |
+| `viewport.js` | 280 | Transforms, zoom, pan, touch, resize, hit-testing, ctrl/shift drag |
+| `popover.js` | 140 | `openPopover()`, `closePopover()`, font/size/pos/gap/text controls |
+| `solver.js` | 200 | SSE solve engine: `startSolve()`, `stopSolve()`, `handleSolveEvent()`, `acceptSolution()` |
+| `associates.js` | 130 | `matchAssociates()`, tier badges, `showAssocDetail()`, `isVictimMatch()` |
+| `main.js` | 170 | Entry point: upload, drag-drop, page nav, font/associate loading, init wiring |
+
+### Agent Visibility Optimizations
+
+- **Small, focused files** — each module fits comfortably in a single agent context read
+- **`types.js` as source of truth** — an agent reads this first to understand all data shapes
+- **Explicit imports/exports** — every module declares what it provides and consumes
+- **One-line module header** — e.g. `/** Canvas rendering — draws redaction overlays on the document image. */`
+- **Named DOM imports** — each module imports only the elements it touches, making dependencies obvious
+
+## Dependency Graph
+
+```
+types.js          (no imports)
+state.js          ← types.js
+dom.js            (no imports)
+canvas.js         ← state.js, dom.js
+viewport.js       ← state.js, dom.js, canvas.js
+associates.js     ← state.js, dom.js
+popover.js        ← state.js, dom.js, canvas.js
+solver.js         ← state.js, dom.js, canvas.js, associates.js, popover.js
+main.js           ← state.js, dom.js, canvas.js, viewport.js, popover.js, solver.js
+```
+
+No circular dependencies. The one tricky spot (`closePopover` calls `stopSolve`, `acceptSolution` calls `closePopover`) is resolved via a callback registration pattern:
+
+```js
+// popover.js
+let _onClose = null;
+export function setOnPopoverClose(fn) { _onClose = fn; }
+export function closePopover() {
+  // ... hide elements ...
+  if (_onClose) _onClose();
+}
+
+// main.js
+import { setOnPopoverClose } from './popover.js';
+import { stopSolve } from './solver.js';
+setOnPopoverClose(stopSolve);
+```
+
+## Type Safety
+
+Every file starts with `// @ts-check`. Key type definitions in `types.js`:
+
+```js
+/** @typedef {Object} Redaction
+ * @property {string} id
+ * @property {number} x
+ * @property {number} y
+ * @property {number} w
+ * @property {number} h
+ * @property {number} page
+ * @property {'unanalyzed'|'analyzing'|'analyzed'|'solved'|'error'} status
+ * @property {Analysis|null} analysis
+ * @property {Solution|null} solution
+ * @property {string|null} preview
+ * @property {Overrides} [overrides]
+ */
+
+/** @typedef {Object} Analysis ... */
+/** @typedef {Object} Overrides ... */
+/** @typedef {Object} Solution ... */
+/** @typedef {Object} AppState ... */
+/** @typedef {Object} Font ... */
+/** @typedef {Object} AssociatesData ... */
+```
+
+## Editor Configuration
+
+A `jsconfig.json` in `unredact/static/`:
+
+```json
+{
+  "compilerOptions": {
+    "checkJs": true,
+    "strict": true,
+    "target": "ES2022",
+    "module": "ES2022"
+  },
+  "include": ["./**/*.js"]
+}
+```
+
+## HTML Change
+
+```html
+<!-- before -->
+<script src="/static/app.js"></script>
+
+<!-- after -->
+<script type="module" src="/static/main.js"></script>
+```
+
+## Backend Changes
+
+None. FastAPI's `StaticFiles` mount serves all `.js` files. ES module imports resolve via HTTP requests automatically.
+
+## Verification
+
+- All existing functionality works identically after the split
+- No new dependencies, no `package.json`, no `node_modules`
+- VS Code shows type errors via `@ts-check` without any build command