Fix selectedRepositories undefined error with generic filter validation

[michelroegl-brunner]

Fixes/Prevents issues like #320 in the future.

Problem

Users were experiencing TypeError: can't access property "length", selectedRepositories is undefined when loading saved filters that were created before the selectedRepositories property was added.

Solution

Created a generic filter validation utility that:

• Provides a single source of truth for default filter values
• Merges saved filters with defaults, ensuring all properties exist
• Prevents crashes when loading old saved filters missing new properties
• Future-proofs against similar issues when new filter properties are added

Changes

• Created filterUtils.ts with getDefaultFilters() and mergeFiltersWithDefaults() functions
• Updated ScriptsGrid.tsx, DownloadedScriptsTab.tsx, and FilterBar.tsx to use the utility functions
• All filter loading now uses mergeFiltersWithDefaults() to ensure complete FilterState objects

Benefits

• ✅ Fixes the immediate crash issue
• ✅ Type-safe: TypeScript ensures all properties are covered
• ✅ Future-proof: New filter properties automatically get defaults
• ✅ No defensive checks needed throughout the codebase
• ✅ Single source of truth for default values