feat: implement hierarchical submenu support for BitBar/Argos format

[insign]

Summary

Implements support for nested submenus in the system tray menu using the BitBar/Argos format where lines prefixed with -- indicate submenu items.

This feature enables plugins to create complex hierarchical menus, just like BitBar and Argos have always supported.

Changes

Parser (lib/core/output_parser.dart)

• Modified _parseBitBar() to detect -- prefixes and build hierarchical MenuItem structures
• Added _parseIndentedLine() to calculate depth from -- prefix count (-- = level 1, ---- = level 2, etc.)
• Added _parseMenuItemFromLine() for cleaner individual line parsing
• Fixed separator logic: first --- marks menu start, subsequent separators add visual dividers

Tray Service (lib/services/tray_service.dart)

• Modified _updateMenu() to recursively convert plugin model MenuItems to tray_manager MenuItems
• Added _convertMenuItems() for recursive conversion with submenu support
• Used MenuItem.submenu() factory for proper tray submenu rendering
• Plugins with menu items now show as expandable submenus in the tray

Tests

• Added comprehensive unit tests covering 1-5 levels of nesting
• Tests for submenu with attributes (bash, href, color)
• Tests for mixed depth levels
• All existing tests continue to pass (backward compatibility)

Documentation & Examples

• Created plugins/examples/submenu_demo.30s.sh demonstrating submenu usage
• Documentation already existed in docs/plugin-development.md (lines 183-195)

Example Usage

echo "📊 System"
echo "---"
echo "Hardware"
echo "--CPU"
echo "----Model: Intel i9"
echo "----Cores: 12"
echo "--Memory"
echo "----Used: 8GB"
echo "----Free: 24GB"

Testing

• ✅ make analyze - No issues
• ✅ make coverage - 45.8% (target: 35-60%)
• ✅ make linux - Build successful
• ✅ make android - Build successful

Closes #37