feat: drawing canvas with brush tool (Fixes #93)

[zxnb01]

Fixes #93

🎨 Drawing Canvas Implementation
This PR implements a comprehensive drawing system for the text editing app as requested in #93.

Features Implemented:
Single brush feature with advanced drawing capabilities
Brush size slider with intuitive controls and real-time preview
Comprehensive color palette with color picker and preset options
Improved brush size UI - updated to a better, more user-friendly design

Functionality Ensured:
Drawing files save correctly - all drawing data persists properly when saving pages
Text compatibility maintained - texts remain fully draggable and editable on the drawing screen
Seamless integration - text and drawing features work together without conflicts
Enhanced page previews - saved pages display properly with both drawings and texts intact

UI Improvements:
Better brush size interface - refined the brush size controls for improved user experience
Unified canvas - smooth interaction between text editing and drawing modes

Technical Implementation:

• Uses Flutter's native CustomPainter and Canvas API
• No external drawing libraries required
• Fixed save/load issues - drawings maintain stroke appearance
• Proper JSON serialization of drawing paths and properties

[github-actions]

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[may-tas]

@zxnb01 instead of adding all the different sub classes inside a single file drawing_canvas.dart, make a subfolder inside widgets and add the classes in different files.

[zxnb01]

done ,u can review now

[may-tas]

@zxnb01
Upload a working screen recording to check that nothing is broken. In the recording, please use both the text and drawing modes, as I want to see if they work well together.

[zxnb01]

Screen.Recording.2025-09-24.164239.mp4

here is the demo.

[may-tas]

@zxnb01
Please pull the latest changes, as this PR has been merged. All colors have now been centralized, so make sure to use this colors class wherever necessary.
#95

[zxnb01]

@may-tas resolved the merge conflicts in cubit folder
and added a seperate subfolder for the monolith drawing.dart
can u recheck

[may-tas]

@zxnb01
You should utilize the ColorConstants class for all necessary colors and functions to apply different shades using alpha values. Please update all files where hardcoded colors have been used and replace them with ColorConstants.

[zxnb01]

@may-tas made the changes as requested.

Screen.Recording.2025-09-25.160619.mp4

final video to ensure there is no breakage in the code.

[may-tas]

LGTM, Merging.
Thanks for contributing.