Migrate from yarn to pnpm

Replace yarn with pnpm as the package manager for better performance
and more efficient disk space usage through content-addressable storage.

Changes:
- Replace yarn.lock with pnpm-lock.yaml
- Update all Makefile commands to use pnpm instead of yarn
- Update .gitignore to include pnpm-specific files
- Add pnpm-debug.log* and .pnpm-store to .gitignore
- Update clean target in Makefile to remove pnpm-lock.yaml

All tests passing (61 tests across 5 test files).
Build verified to work with pnpm.

Author: claude

.gitignore

@@ -21,6 +21,10 @@
 npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
+pnpm-debug.log*
 
 # TypeScript
 *.tsbuildinfo
+
+# pnpm
+.pnpm-store

Makefile

@@ -18,48 +18,49 @@ help:
 
 # Install dependencies
 install:
-	yarn install --frozen-lockfile
+	pnpm install --frozen-lockfile
 
 # Start Vite development server
 dev:
-	yarn dev
+	pnpm dev
 
 # Alias for dev
 start: dev
 
 # Run tests with Vitest
 test:
-	yarn test --run
+	pnpm test --run
 
 # Run tests with Vitest UI
 test-ui:
-	yarn test:ui
+	pnpm test:ui
 
 # Run tests with coverage
 test-coverage:
-	yarn test:coverage --run
+	pnpm test:coverage --run
 
 # Build production bundle
 build:
-	yarn build
+	pnpm build
 
 # Preview production build
 preview:
-	yarn preview
+	pnpm preview
 
 # Check code formatting with Prettier
 lint:
-	yarn prettier --check "src/**/*.{js,jsx,ts,tsx,json,css,scss,md}"
+	pnpm prettier --check "src/**/*.{js,jsx,ts,tsx,json,css,scss,md}"
 
 # Fix code formatting with Prettier
 lint-fix:
-	yarn prettier --write "src/**/*.{js,jsx,ts,tsx,json,css,scss,md}"
+	pnpm prettier --write "src/**/*.{js,jsx,ts,tsx,json,css,scss,md}"
 
 # Clean build artifacts
 clean:
 	rm -rf build
 	rm -rf coverage
 	rm -rf node_modules
+	rm -f pnpm-lock.yaml
 
 # Run all CI checks
 ci: test-coverage build lint