Migrate icon database maintenance to a JSON-first workflow

[Marc-André Moreau (mamoreau-devolutions)]

Summary

This PR moves icon database maintenance to a JSON-first workflow centered on WebBasedData/screenshot-database-v2.json.

It removes the legacy XLSX source and stale invalid URL snapshot, rewrites the maintenance script around JSON validation and count synchronization, adds a Markdown gallery exporter for visual review, and updates the database contents to fix truncated package names, add explicit Devolutions WinGet entries, and remove obvious dead URLs.

Changes

• make WebBasedData/screenshot-database-v2.json the source of truth
• delete WebBasedData/screenshot_database.xlsx
• delete WebBasedData/invalid_urls.txt
• rewrite scripts/update-icons.ps1 for JSON sync and validation
• make update-icons validation tolerate a missing invalid_urls.txt file
• add scripts/export-icon-database-gallery.ps1 with include/exclude package filters
• repair icon database entries, including explicit Devolutions package coverage
• clean up obvious dead icon and screenshot URLs
• resync package_count with the current JSON payload

Validation

• parsed WebBasedData/screenshot-database-v2.json as JSON using ConvertFrom-Json -AsHashtable
• deserialized the database with System.Text.Json into the same structure UniGetUI uses at runtime
• verified package_count matches the current icons_and_screenshots contents
• ran ./scripts/update-icons.ps1 -Validate -MaxPackages 1

Notes

The validation script still reports warnings for __test_entry_DO_NOT_EDIT_PLEASE because those URLs are intentionally fake placeholders used for testing.