fix(settings): repair legacy path settings that block every settings save

A subJsonPath (or subPath/subClashPath/webBasePath) stored without its
leading/trailing slash — written before the slash rules existed, or
restored from an old backup — fails the frontend's whole-form validation,
so every save on the Settings page is rejected client-side. The backend's
CheckValid would normalize the value, but a save request never reaches it,
leaving the panel wedged until someone edits the database by hand.

Normalize the stored path rows at startup, mirroring CheckValid's slash
rules. The pass is idempotent and not seeder-gated, since a restored
backup can reintroduce bad values at any time.

Also add the missing pages.settings.validation.pathLeadingSlash key to
all 13 locales — the validation error used to render as its raw key.

Closes #5726

internal/database/db_seed_test.go

@@ -153,3 +153,44 @@ func TestNormalizeInboundClientSubId_FillsMissingAndPreservesExisting(t *testing
 		t.Fatalf("expected one InboundClientSubIdFix history row, got %d", historyCount)
 	}
 }
+
+func TestNormalizeSettingPaths_RepairsLegacyValues(t *testing.T) {
+	dbDir := t.TempDir()
+	t.Setenv("XUI_DB_FOLDER", dbDir)
+	if err := InitDB(filepath.Join(dbDir, "x-ui.db")); err != nil {
+		t.Fatalf("InitDB failed: %v", err)
+	}
+	t.Cleanup(func() { _ = CloseDB() })
+
+	seed := []model.Setting{
+		{Key: "subJsonPath", Value: "YIrCXJOOOL"},
+		{Key: "subPath", Value: "/sub"},
+		{Key: "subClashPath", Value: "clash/"},
+		{Key: "webBasePath", Value: "/panel/"},
+	}
+	for i := range seed {
+		if err := db.Create(&seed[i]).Error; err != nil {
+			t.Fatalf("seed setting %s: %v", seed[i].Key, err)
+		}
+	}
+
+	if err := normalizeSettingPaths(); err != nil {
+		t.Fatalf("normalizeSettingPaths: %v", err)
+	}
+
+	want := map[string]string{
+		"subJsonPath":  "/YIrCXJOOOL/",
+		"subPath":      "/sub/",
+		"subClashPath": "/clash/",
+		"webBasePath":  "/panel/",
+	}
+	for key, expected := range want {
+		var row model.Setting
+		if err := db.Where("key = ?", key).First(&row).Error; err != nil {
+			t.Fatalf("read %s: %v", key, err)
+		}
+		if row.Value != expected {
+			t.Errorf("%s = %q, want %q", key, row.Value, expected)
+		}
+	}
+}