Add PDF documentation export, BOM cost estimation, smart search keywords, and UI improvements

[RolandWa]

Summary

This PR adds several quality-of-life improvements to the JLCPCB tools plugin, focused on better documentation output, smarter part search, cost visibility, and improved part list readability.

New Features

• PCB & Schematic PDF export — New generate_pcb_pdf() and generate_schematic_pdf() methods produce documentation PDFs alongside the standard Gerber/BOM/CPL output, stored in jlcpcb/documentation/
• BOM cost estimation — calculate_bom_cost() and update_bom_cost() parse LCSC pricing data to provide a real-time cost estimate in the main window
• Auto-generate search keywords — New setting to automatically derive search keywords from reference designator prefix (R, C, D, Q, U) and component value, streamlining part lookup
• Assign part to all same-value components — assign_to_all_same_value() lets users assign an LCSC part to all footprints sharing the same value in one action
• Component category detection — get_component_category() and extract_package_size() helpers for smarter part matching

Improvements

• Row coloring in part list — Color-coded rows in the DataViewCtrl: grey for BOM-excluded, red for missing LCSC, yellow for low stock (<100), green for high stock (>10,000)
• Smarter search result sorting — Stock-prioritized sorting: in-stock parts appear first; secondary sort by stock when using other sort columns
• Simplified position offset calculation — Removed complex trigonometric offset reposition logic in favor of direct offset application
• Removed paste layers from default plot plan — Paste top/bottom layers no longer included by default (not needed for JLCPCB fabrication)
• Auto-include User.Cuts layers — Any *.Cuts* layers are now automatically added to the Gerber plot plan

Housekeeping

• Version bump to 2026.04.1
• Added .github/copilot-instructions.md for AI-assisted development

Testing

Tested with KiCad v9 on Windows. All existing pytest common/ tests pass.