feat(commands): add create button and file browser to commands page

- Add "New Command" button to CommandsManagerPage header
- Integrate CommandCreateDialog for creating/importing commands
- Add file browser button to source path input in CommandCreateDialog
  - Uses FloatingFileBrowser component for file selection
  - Supports browsing project directory for .md command files
- Add i18n keys for browse file button (en/zh)

Users can now create commands via:
1. Import existing .md file (with visual file picker)
2. AI-generate new command from description

ccw/frontend/src/locales/en/commands.json

@@ -74,6 +74,7 @@
     "sourcePath": "Source File Path",
     "sourcePathPlaceholder": "Enter absolute path to command file",
     "sourcePathHint": "File must be a valid command markdown file",
+    "browseFile": "Browse files",
     "customName": "Custom Name",
     "customNamePlaceholder": "Leave empty to use original name",
     "customNameHint": "Optional, overrides default command name",