feat(ui): file browser and connection string builder dialogs (#44)

Enter on File Path inputs opens a file browser (load/save, extension
filter, hidden toggle, overwrite confirm). Enter on Connection String
inputs opens a builder for PostgreSQL, MSSQL and SQLite with masked
password/preview, parsing and optional connection test.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

pkg/ui/connstring_check.go

@@ -0,0 +1,62 @@
+package ui
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+	_ "github.com/microsoft/go-mssqldb"
+	_ "modernc.org/sqlite"
+)
+
+// connTestTimeout bounds how long "Test connection" may block.
+const connTestTimeout = 5 * time.Second
+
+// TestConnection opens and pings the database described by f. Any occurrence
+// of the password in the returned error is masked.
+func TestConnection(f ConnFields) error {
+	ctx, cancel := context.WithTimeout(context.Background(), connTestTimeout)
+	defer cancel()
+
+	err := testConnection(ctx, f)
+	if err != nil && f.Password != "" {
+		err = fmt.Errorf("%s", strings.ReplaceAll(err.Error(), f.Password, maskedPassword))
+	}
+	return err
+}
+
+func testConnection(ctx context.Context, f ConnFields) error {
+	switch f.Kind {
+	case ConnPostgres:
+		conn, err := pgx.Connect(ctx, BuildConnString(f, false))
+		if err != nil {
+			return err
+		}
+		return conn.Close(ctx)
+	case ConnMSSQL:
+		return pingSQL(ctx, "sqlserver", BuildConnString(f, false))
+	case ConnSQLite:
+		if f.FilePath == "" {
+			return fmt.Errorf("file path is required")
+		}
+		// Opening a missing SQLite file would silently create it.
+		if _, err := os.Stat(f.FilePath); err != nil {
+			return err
+		}
+		return pingSQL(ctx, "sqlite", f.FilePath)
+	}
+	return fmt.Errorf("unsupported connection type %q", f.Kind)
+}
+
+func pingSQL(ctx context.Context, driver, dsn string) error {
+	db, err := sql.Open(driver, dsn)
+	if err != nil {
+		return err
+	}
+	defer db.Close()
+	return db.PingContext(ctx)
+}