feat(go): add --dry-run to fuj sync

Mirror fuj infer's read-only mode: SyncOpts.DryRun skips WriteHeader,
AppendValues, and SortByDateColumn, printing one "Dry run: would …"
line per planned operation instead. ID-dedup still runs so the output
reflects exactly what the next real sync would write.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/cmd/fuj/main.go

@@ -134,8 +134,9 @@ func syncCmd(args []string) {
 	fromStr := fs.String("from", "", "start date YYYY-MM-DD")
 	toStr := fs.String("to", "", "end date YYYY-MM-DD")
 	sort := fs.Bool("sort", true, "sort sheet by date after appending")
+	dryRun := fs.Bool("dry-run", false, "print planned writes without modifying the sheet")
 	fs.Usage = func() {
-		fmt.Fprintln(os.Stderr, "usage: fuj sync [--days N] [--from YYYY-MM-DD --to YYYY-MM-DD] [--sort]")
+		fmt.Fprintln(os.Stderr, "usage: fuj sync [--days N] [--from YYYY-MM-DD --to YYYY-MM-DD] [--sort] [--dry-run]")
 		fs.PrintDefaults()
 	}
 	if err := fs.Parse(args); err != nil {
@@ -153,7 +154,7 @@ func syncCmd(args []string) {
 	}
 	fioCli := fio.New(cfg.FioAPIToken, config.IBANAccountNum(cfg.BankAccount), nil)
 
-	opts := banksync.SyncOpts{Days: *days, Sort: *sort}
+	opts := banksync.SyncOpts{Days: *days, Sort: *sort, DryRun: *dryRun}
 	if *fromStr != "" && *toStr != "" {
 		opts.From, err = time.Parse("2006-01-02", *fromStr)
 		if err != nil {
@@ -172,7 +173,11 @@ func syncCmd(args []string) {
 		fmt.Fprintf(os.Stderr, "fuj sync: %v\n", err)
 		os.Exit(1)
 	}
-	fmt.Printf("Synced %d new transaction(s).\n", n)
+	if *dryRun {
+		fmt.Printf("Dry run: would sync %d new transaction(s).\n", n)
+	} else {
+		fmt.Printf("Synced %d new transaction(s).\n", n)
+	}
 }
 
 func inferCmd(args []string) {

go/internal/services/banksync/sync.go

@@ -30,6 +30,7 @@ type SyncOpts struct {
 	Days     int       // look-back window when From/To are zero
 	From, To time.Time // explicit window (overrides Days)
 	Sort     bool      // sort the sheet by Date after appending
+	DryRun   bool      // print planned writes without modifying the sheet
 }
 
 // SyncToSheets fetches Fio transactions and appends new ones to the payments sheet.
@@ -52,8 +53,12 @@ func SyncToSheets(
 	if len(rows) > 0 {
 		header := rows[0]
 		if !headerMatches(header) {
-			if err := sh.WriteHeader(ctx, spreadsheetID, columnLabels); err != nil {
-				return 0, fmt.Errorf("sync: write header: %w", err)
+			if opts.DryRun {
+				fmt.Println("Dry run: would write header row")
+			} else {
+				if err := sh.WriteHeader(ctx, spreadsheetID, columnLabels); err != nil {
+					return 0, fmt.Errorf("sync: write header: %w", err)
+				}
 			}
 		} else {
 			for _, row := range rows[1:] {
@@ -113,6 +118,17 @@ func SyncToSheets(
 		return 0, nil
 	}
 
+	if opts.DryRun {
+		for _, row := range newRows {
+			fmt.Printf("Dry run: would append date=%v amount=%v sender=%v vs=%v message=%v\n",
+				row[0], row[1], row[6], row[7], row[8])
+		}
+		if opts.Sort {
+			fmt.Println("Dry run: would sort by date")
+		}
+		return len(newRows), nil
+	}
+
 	if err := sh.AppendValues(ctx, spreadsheetID, "A2", newRows); err != nil {
 		return 0, fmt.Errorf("sync: append: %w", err)
 	}

go/internal/services/banksync/sync_test.go

@@ -127,6 +127,23 @@ func TestSyncToSheets_ExplicitDateWindow(t *testing.T) {
 	}
 }
 
+func TestSyncToSheets_DryRun(t *testing.T) {
+	sh := &sheets.Fake{Values: map[string][][]any{"SHEETID/A1:K": {}}}
+	fioFake := &fio.Fake{Transactions: testFioTxns}
+
+	n, err := SyncToSheets(context.Background(), "SHEETID", fioFake, sh,
+		SyncOpts{Days: 30, Sort: true, DryRun: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 2 {
+		t.Errorf("want 2 planned, got %d", n)
+	}
+	if len(sh.Appended) != 0 {
+		t.Error("dry-run must not call AppendValues")
+	}
+}
+
 // syncIDFor mirrors what SyncToSheets computes for a given fio.Transaction.
 func syncIDFor(tx fio.Transaction) string {
 	currency := tx.Currency