fix: support diacritic-insensitive search indexing closes (#629)

[itzabdoull]

Closes #629

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Description
This PR resolves issue #629 where search queries containing or targeting courses containing Unicode diacritics (accents/umlauts/etc.) were not matching. For example, searching for "cafe" now successfully returns courses titled "Café Communication", and "resume" finds "Résumé Writing".

Key Changes
New Utility Function: Added

normalizeText
in src/utils/stringUtils.ts to convert text into Unicode NFD form and strip out all diacritical marks via regex (normalize('NFD').replace(/[\u0300-\u036f]/g, '')).
Normalized Indexing (Index-Build Time): Integrated normalizeText into the course indexing pipeline in

searchIndex.ts
for:
Course title tokens and individual word suggestions.
Course category tokens.
Instructor name tokens.
Cap-sliced course description tokens.
Note: Original display text in documents is preserved unchanged so the user interface renders unmodified characters (e.g., 'Café').
Normalized Query Matching (Query Time): Normalized the user query string in the search function before tokenization and prefix matching.
Unit Tests: Added

searchIndex.test.ts
covering matches for:
'Café' → 'cafe'
'Résumé' → 'resume'
'München' → 'munchen'
'Ñoño' → 'nono'
Verification
Run npx jest tests/services/searchIndex.test.ts (All 4 unit tests pass)
Run npx jest tests/services/searchIndex.perf.test.ts (Validates execution meets the 50ms index query budget)

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