Add side panels, task selection, graph animation, and project docs

- Foldable left panel (user profile) and right panel (task details)
- Clicking a task in the list or graph node selects it and shows details
- Both views (task list + graph) always mounted via absolute inset-0 for
  correct canvas dimensions; tabs toggle visibility with opacity
- Graph node selection animation: other nodes repel outward (charge -600),
  then selected node smoothly slides to center (500ms cubic ease-out),
  then charge restores to -120 and graph stabilizes
- Graph re-fits on tab switch and panel resize via ResizeObserver
- Fix UUID string IDs throughout (backend returns UUIDs, not integers)
- Add TaskDetailPanel, UserPanel components
- Add CLAUDE.md project documentation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/client-only/error.js

@@ -0,0 +1,4 @@
+throw new Error(
+  "This module cannot be imported from a Server Component module. " +
+    "It should only be used from a Client Component."
+);

frontend/node_modules/client-only/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "client-only",
+  "description": "This is a marker package to indicate that a module can only be used in Client Components.",
+  "keywords": [
+    "react"
+  ],
+  "version": "0.0.1",
+  "homepage": "https://reactjs.org/",
+  "bugs": "https://github.com/facebook/react/issues",
+  "license": "MIT",
+  "files": ["index.js", "error.js"],
+  "main": "index.js",
+  "exports": {
+    ".": {
+      "react-server": "./error.js",
+      "default": "./index.js"
+    }
+  }
+}