docs: add CLAUDE.md with project architecture and dev commands

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

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Tusk is a PostgreSQL database management GUI built with Tauri 2 (Rust backend) and React (TypeScript frontend).
+
+## Development Commands
+
+```bash
+npm run tauri dev     # Start full app in dev mode (Vite HMR + Rust backend)
+npm run dev           # Start only the Vite frontend dev server (port 5173)
+npm run build         # Build frontend (tsc + vite build)
+npm run tauri build   # Build production desktop executable
+npm run lint          # ESLint
+```
+
+Rust backend builds automatically via Tauri when running `npm run tauri dev/build`. To check Rust compilation independently:
+```bash
+cd src-tauri && cargo check
+```
+
+## Architecture
+
+```
+Frontend (React/TS) ──IPC invoke()──> Backend (Rust/Tauri) ──sqlx──> PostgreSQL
+```
+
+### IPC Communication Pattern
+
+All frontend-backend communication goes through Tauri's typed IPC. The full pipeline for adding a new backend-exposed feature:
+
+1. **Rust command** — `#[tauri::command]` async fn in `src-tauri/src/commands/<domain>.rs`
+2. **Register** — add to `invoke_handler` in `src-tauri/src/lib.rs`
+3. **TS wrapper** — typed `invoke()` call in `src/lib/tauri.ts`
+4. **React hook** — TanStack React Query hook in `src/hooks/use-<domain>.ts`
+5. **Types** — shared interfaces in `src/types/index.ts`
+
+### Backend (src-tauri/src/)
+
+- **`state.rs`** — `AppState`: holds `RwLock<HashMap<connection_id, PgPool>>`, read-only flags, config path. Connections default to read-only.
+- **`error.rs`** — `TuskError` enum with `thiserror`, serialized as string for IPC. `TuskResult<T>` type alias.
+- **`utils.rs`** — `escape_ident()` for safe SQL identifier quoting.
+- **`commands/`** — one module per domain (connections, queries, schema, data, export, management, history, saved_queries). Each function takes `State<'_, AppState>` and returns `TuskResult<T>`.
+- **`models/`** — serde-serializable structs matching TypeScript types.
+
+SQL safety: identifiers use `escape_ident()`, data queries use sqlx parameterized queries. Read-only mode wraps queries in `SET TRANSACTION READ ONLY` + `ROLLBACK`.
+
+### Frontend (src/)
+
+- **State**: Zustand store (`stores/app-store.ts`) — connections, active connection/database, tabs, read-only flags, pg version.
+- **Data fetching**: TanStack React Query hooks in `hooks/` — one hook per domain, wrapping `src/lib/tauri.ts` functions.
+- **UI**: shadcn/ui + Radix primitives, Tailwind CSS 4, dark mode via next-themes. SQL editor uses CodeMirror.
+- **Layout**: resizable panels (sidebar + main area with tab bar).
+
+### Stored Data
+
+Connections are persisted to `~/.config/tusk/connections.json`. History and saved queries are stored in similar JSON files in the same config directory.
+
+## Conventions
+
+- **Frontend files**: Components `PascalCase.tsx`, hooks `use-kebab-case.ts`, one feature per directory under `components/`
+- **Path alias**: `@` maps to `./src`
+- **Rust errors**: always use `TuskError` variants, never `unwrap()` in commands
+- **New PG types**: add conversion case in `pg_value_to_json()` in `commands/queries.rs`