Fix manifest icon references and add file reference linter

Manifest fixes:
- Updated all icon references from .png to .svg files
- Changed MIME types from image/png to image/svg+xml
- Removed missing screenshots section
- Fixed HTML favicon references to use SVG files

New reference linter (lint-references.py):
- Validates all file references in HTML and manifest files
- Checks that referenced files actually exist
- Prevents broken links and missing assets
- Integrated into justfile as 'validate-references'
- Added to check-all command for comprehensive validation

Justfile enhancements:
- Added validate-references command
- Updated check-all to include reference validation
- Enhanced validate command with reference checking

Documentation updates:
- Updated CLAUDE.md with reference validation info
- Added file reference checking to code quality standards

This prevents issues where manifest/HTML reference non-existent files,
improving PWA reliability and catching asset mismatches early.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -68,6 +68,9 @@ bun run check             # Check formatting without modifying
 # Python
 black server.py           # Format Python code
 pylint server.py          # Lint Python code
+
+# File References
+just validate-references  # Check HTML/manifest file references exist
 ```
 
 ### Icon Generation
@@ -164,10 +167,11 @@ node app/create-icons.js  # Generate PNG icons
    - Print styles
 
 ### Code Quality
-- ESLint: No errors, minimal warnings
+- ESLint: No errors, no warnings
 - Prettier: All files formatted
 - Black: Python code formatted
 - Pylint: Score 10/10
+- File References: All HTML/manifest references exist
 
 ## Common Tasks