Fix cursor positioning, resize, and paste; add App tests and Cursor rules

- Use CoreGraphics (core-graphics crate) for global mouse position instead
  of Tauri's window-relative cursor_position() which fails when hidden
- Switch to titleBarStyle overlay with hiddenTitle for native resize handles
  while keeping the frameless look (decorations:false had no resize affordance)
- Fix paste_and_refocus: use .output() instead of .spawn() so osascript
  actually completes, increase delay to 250ms for reliable app refocus
- Add comprehensive App.test.tsx (7 integration tests) bringing total to 47
- Add multi-select and type badge tests for ClipboardList and ContextMenu
- Update Tauri mock in setup.ts with innerSize/scaleFactor for resize tests
- Create .cursor/rules/ with 4 rule files for project conventions
- Add npm run lint and npm run check scripts
- Update README with full usage table and current test counts (74 total)

Co-authored-by: Cursor <cursoragent@cursor.com>

.cursor/rules/testing.mdc

@@ -0,0 +1,29 @@
+---
+description: Testing conventions for Rust and TypeScript
+globs: "**/*.test.{ts,tsx}"
+alwaysApply: false
+---
+
+# Testing Conventions
+
+## Rust Tests (cargo test)
+
+- In-module `#[cfg(test)]` blocks using `Database::in_memory()`
+- Test naming: `snake_case` describing behavior (e.g. `trim_preserves_pinned_entries`)
+- All tests must pass before committing: `cd src-tauri && cargo test`
+
+## Frontend Tests (Vitest)
+
+- Use `@testing-library/react` with `jsdom` environment
+- Mock Tauri APIs in `src/test/setup.ts` — never call real IPC in tests
+- Use `makeEntry()` and `makeSettings()` factories for test data
+- Call `resetIdCounter()` in `beforeEach` when IDs matter
+- When testing async Tauri commands, use `waitFor()` assertions
+- Guard `scrollIntoView` with optional chaining (`el?.scrollIntoView?.()`) since jsdom doesn't implement it
+
+## Running Tests
+
+```bash
+cd src-tauri && cargo test    # Rust: 27 tests
+npx vitest run                # Frontend: 47 tests
+```