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>

backend/Cargo.toml

@@ -0,0 +1,27 @@
+[package]
+name = "taskpile-backend"
+version = "0.1.0"
+edition = "2021"
+
+[[bin]]
+name = "taskpile-backend"
+path = "src/main.rs"
+
+[lib]
+name = "taskpile_backend"
+path = "src/lib.rs"
+
+[dependencies]
+axum = "0.7"
+tokio = { version = "1", features = ["full"] }
+sqlx = { version = "0.7", features = ["runtime-tokio-rustls", "sqlite"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tower-http = { version = "0.5", features = ["cors"] }
+uuid = { version = "1", features = ["v4"] }
+rand = "0.8"
+anyhow = "1"
+
+[dev-dependencies]
+axum-test = "14"
+tower = "0.4"