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

plugin/filter/render.go

@@ -454,6 +454,11 @@ func (r *renderer) renderContainsCondition(cond *ContainsCondition) (renderResul
 	column := field.columnExpr(r.dialect)
 	arg := fmt.Sprintf("%%%s%%", cond.Value)
 	switch r.dialect {
+	case DialectSQLite:
+		// Use custom Unicode-aware case folding function for case-insensitive comparison.
+		// This overcomes SQLite's ASCII-only LOWER() limitation.
+		sql := fmt.Sprintf("memos_unicode_lower(%s) LIKE memos_unicode_lower(%s)", column, r.addArg(arg))
+		return renderResult{sql: sql}, nil
 	case DialectPostgres:
 		sql := fmt.Sprintf("%s ILIKE %s", column, r.addArg(arg))
 		return renderResult{sql: sql}, nil

store/db/sqlite/functions.go

@@ -0,0 +1,44 @@
+// Package sqlite provides SQLite driver implementation with custom functions.
+// Custom functions are registered globally on first use to extend SQLite's
+// limited ASCII-only text operations with proper Unicode support.
+package sqlite
+
+import (
+	"database/sql/driver"
+	"sync"
+
+	"golang.org/x/text/cases"
+	msqlite "modernc.org/sqlite"
+)
+
+var (
+	registerUnicodeLowerOnce sync.Once
+	registerUnicodeLowerErr  error
+	// unicodeFold provides Unicode case folding for case-insensitive comparisons.
+	// It's safe to use concurrently and reused across all function calls.
+	unicodeFold = cases.Fold()
+)
+
+// ensureUnicodeLowerRegistered registers the memos_unicode_lower custom function
+// with SQLite. This function provides proper Unicode case folding for case-insensitive
+// text comparisons, overcoming modernc.org/sqlite's lack of ICU extension.
+//
+// The function is registered once globally and is safe to call multiple times.
+func ensureUnicodeLowerRegistered() error {
+	registerUnicodeLowerOnce.Do(func() {
+		registerUnicodeLowerErr = msqlite.RegisterScalarFunction("memos_unicode_lower", 1, func(_ *msqlite.FunctionContext, args []driver.Value) (driver.Value, error) {
+			if len(args) == 0 || args[0] == nil {
+				return nil, nil
+			}
+			switch v := args[0].(type) {
+			case string:
+				return unicodeFold.String(v), nil
+			case []byte:
+				return unicodeFold.String(string(v)), nil
+			default:
+				return v, nil
+			}
+		})
+	})
+	return registerUnicodeLowerErr
+}

store/db/sqlite/sqlite.go

@@ -6,8 +6,8 @@ import (
 
 	"github.com/pkg/errors"
 
-	// Import the SQLite driver.
-	_ "modernc.org/sqlite"
+	// Note: modernc.org/sqlite driver is imported in functions.go where
+	// RegisterScalarFunction is used. No blank import needed here.
 
 	"github.com/usememos/memos/internal/profile"
 	"github.com/usememos/memos/store"
@@ -27,6 +27,10 @@ func NewDB(profile *profile.Profile) (store.Driver, error) {
 		return nil, errors.New("dsn required")
 	}
 
+	if err := ensureUnicodeLowerRegistered(); err != nil {
+		return nil, errors.Wrap(err, "failed to register sqlite unicode lower function")
+	}
+
 	// Connect to the database with some sane settings:
 	// - No shared-cache: it's obsolete; WAL journal mode is a better solution.
 	// - No foreign key constraints: it's currently disabled by default, but it's a

store/test/memo_filter_test.go

@@ -61,6 +61,17 @@ func TestMemoFilterContentUnicode(t *testing.T) {
 	require.Len(t, memos, 1)
 }
 
+func TestMemoFilterContentUnicodeCaseFold(t *testing.T) {
+	t.Parallel()
+	tc := NewMemoFilterTestContext(t)
+	defer tc.Close()
+
+	tc.CreateMemo(NewMemoBuilder("memo-unicode-case", tc.User.ID).Content("Привет Мир"))
+
+	memos := tc.ListWithFilter(`content.contains("привет")`)
+	require.Len(t, memos, 1)
+}
+
 func TestMemoFilterContentCaseSensitivity(t *testing.T) {
 	t.Parallel()
 	tc := NewMemoFilterTestContext(t)