feat(image): implement linear-time guided filter for robust document shadow removal

[gzz727007]

🦭 Pull Request Description

Summary of Changes

This PR introduces a major upgrade to the document enhancement module (applyDocumentEnhancement) by implementing a linear-time guided filter for robust shadow removal.

Key technical improvements include:

1. High-Performance Background Estimation: Replaces the legacy approach with a fast guided filter ($O(W \times H)$ time complexity) to efficiently estimate and eliminate uneven lighting and shadows.
2. Text Stroke Decoupling: Improves foreground/background separation to maintain text sharpness and contrast, preventing text edges from blurring or being over-bleached.
3. 1px Grid Line Protection: Introduces a structural protection mechanism to preserve extremely fine details, ensuring that thin table borders (e.g., 1px lines in receipts and invoices) are not broken or lost during the shadow removal process.
4. Single-pass Architecture: Adopts a robust single-pass tone mapping pipeline, avoiding the image degradation and unnatural paper tones caused by multi-iteration approaches.

Type of Change

• 🐛 Bug fix (non-breaking change fixing an issue)
• ✨ New feature (non-breaking change adding functionality)
• ⚡ Performance optimization (rendering, memory, or bundle size)
• 🎨 UI / UX refinement
• 🌐 Internationalization / Translations (new or updated language)
• 📝 Documentation update (README, Guides, Comments)
• 🧪 Test suite enhancement

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🛡️ Privacy & Quality Checklist

• 100% Client-Side Invariant: All code runs strictly in the browser RAM / WebAssembly. No user documents or telemetry data are ever sent over the network.
• Automated Tests: I have verified that all existing tests pass (npm test). All 207 image processing tests passed successfully.
• New Tests Added: I have added unit or E2E tests covering new algorithmic behavior (if applicable).
• Responsive & Cross-Browser: Verified that UI works smoothly across desktop and mobile screens.
• Self-Review: I have performed a self-review of my own code and removed unnecessary console.log statements.

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Related Issues

Closes #