fix: add Unicode case-insensitive search for SQLite (#5559)

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

go.mod

@@ -131,7 +131,7 @@ require (
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
 	github.com/valyala/fasttemplate v1.2.2 // indirect
 	golang.org/x/sys v0.37.0 // indirect
-	golang.org/x/text v0.30.0 // indirect
+	golang.org/x/text v0.30.0
 	golang.org/x/time v0.12.0 // indirect
 	google.golang.org/protobuf v1.36.9
 	gopkg.in/yaml.v3 v3.0.1 // indirect