Add custom memos_unicode_lower() SQLite function to enable proper
case-insensitive text search for non-English languages (Cyrillic,
Greek, CJK, etc.).

Previously, SQLite's LOWER() only worked for ASCII characters due to
modernc.org/sqlite lacking ICU extension. This caused searches for
non-English text to be case-sensitive (e.g., searching 'блины' wouldn't
find 'Блины').

Changes:
- Add store/db/sqlite/functions.go with Unicode case folding function
- Register custom function using golang.org/x/text/cases.Fold()
- Update filter renderer to use custom function for SQLite dialect
- Add test for Unicode case-insensitive search
- Make golang.org/x/text a direct dependency

Fixes #5559