Add custom markers feature and remove streamlit deprecated configurations

[migueljgranizo]

This PR introduces a custom markers feature that allows users to place visual symbols at specific coordinates on maps,
and updates Streamlit components to use the new parameter names to fix deprecation warnings.

Changes

✨ Custom Markers Feature

New Functionality

• Added draw_custom_markers() function in prettymaps/draw.py:493 for rendering custom markers on maps
• Integrated custom markers into the plot() function via custom_markers parameter
• Full preset integration with markers.default_style configuration

Coordinate Format Support
The implementation supports three flexible ways to specify marker locations:

1. Explicit coordinates: {'lat': 52.3676, 'lon': 4.9041, ...}
2. Tuple query: {'query': (52.3676, 4.9041), ...}
3. String query (geocoding): {'query': 'Amsterdam Central Station', ...}

String queries leverage OpenStreetMap Nominatim for automatic geocoding, allowing users to specify locations by name.

Validation & Safety

• Coordinate range validation (lat: -90 to 90, lon: -180 to 180)
• Automatic bounds checking to skip markers outside map perimeter
• Graceful error handling with optional logging for skipped markers

Styling Options
Each marker supports full customization:

• marker: Matplotlib marker style (circle, star, triangle, etc.)
• size: Marker size in points²
• color: Face color (hex or named)
• edgecolor: Edge color
• linewidth: Edge width
• alpha: Transparency (0.0 to 1.0)
• zorder: Rendering order

🎨 Streamlit UI Enhancements

Marker Management Interface (app.py:126-327)

• "Add New Marker" form with flexible location input that auto-detects coordinate vs place name format
• "Manage Existing Markers" section with edit/delete functionality
• Real-time marker counter showing {len(st.session_state.custom_markers)} markers
• Session state management for markers and editing state
• Collapsible expanders to keep UI clean

User Experience Improvements

• Smart location parser: automatically detects if input is coordinates or place name
• Advanced options toggle for edge styling, opacity, and z-order
• Live preview with marker count and bounds warning
• Clean marker display showing name, location, type, and color

🔧 Deprecation Fixes

Updated Streamlit Parameters (app.py:469, 521, 531, 540, 549, 555, 558)

• Replaced deprecated use_container_width=True with width="stretch"
• Affects:
  • Generate map button
  • Download PNG/SVG buttons
  • Image displays (generated map and placeholder)
• Fixes Streamlit v1.37+ deprecation warnings

File Handling Improvements (app.py:521-531)

• Proper file reading with context managers to avoid resource warnings
• Pre-read file data before passing to st.download_button()

📚 Documentation

Comprehensive README Section (README.md:1059-1285)

• Complete feature overview and comparison table (custom markers vs keypoints)
• Usage examples for all three coordinate formats
• Full parameter reference table
• Advanced styling examples
• Validation and bounds checking explanation
• Preset integration guide
• Streamlit frontend user guide

Technical Details

Implementation Location

• prettymaps/draw.py:493-647: Core draw_custom_markers() function
• app.py:29-35: Session state initialization
• app.py:126-327: UI components
• app.py:493: Integration with prettymaps.plot() call

Dependencies

• Uses existing OSMnx geocoding for string queries (ox.geocode())
• GeoPandas for marker geometry management
• Matplotlib for marker rendering

Breaking Changes

None. This is a purely additive feature with backward compatibility maintained.