Add a rule builder screen with a live glob preview

Writing a glob by hand meant guessing what it would catch, then running retag
to find out. The new screen, on 4, puts the glob, account and tag fields on
the left and every still-untagged description on the right, sorted
alphabetically and grouped by description with an occurrence count.

Matches are marked as the glob is typed, along with a count, so the effect of
a rule is visible before it is written. Enter appends it to rules.toml via
config.AppendRule, reloads the engine from disk and retags, so the rows it
caught leave the list immediately.

Rules are appended rather than prepended, keeping the precedence of anything
already in the file. Since the preview only lists transactions no existing
rule has tagged, it reflects that precedence for free.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/config/config.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strconv"
 	"strings"
 
 	"github.com/BurntSushi/toml"
@@ -66,6 +67,76 @@ func LoadRules(root string) (*Rules, error) {
 	return &r, nil
 }
 
+// AppendRule adds a rule to the end of rules.toml, creating the file if it is
+// not there yet. Appending rather than inserting means an existing rule always
+// keeps precedence, since the first match wins.
+//
+// The file is rewritten through a temporary file so a failure part-way cannot
+// leave the user with a truncated config.
+func AppendRule(root string, r Rule) error {
+	if r.Match == "" && r.Counterparty == "" && r.Type == "" {
+		return fmt.Errorf("a rule needs a match, counterparty or type pattern")
+	}
+	if r.Tag == "" && !r.Transfer {
+		return fmt.Errorf("a rule needs a tag or transfer = true")
+	}
+
+	path := filepath.Join(root, RulesFile)
+	existing, err := os.ReadFile(path)
+	if err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("read %s: %w", path, err)
+	}
+
+	var b strings.Builder
+	b.Write(existing)
+	if len(existing) > 0 && !strings.HasSuffix(string(existing), "\n") {
+		b.WriteString("\n")
+	}
+	b.WriteString("\n")
+	b.WriteString(formatRule(r))
+
+	tmp, err := os.CreateTemp(root, ".rules-*.toml")
+	if err != nil {
+		return fmt.Errorf("write %s: %w", path, err)
+	}
+	defer os.Remove(tmp.Name())
+
+	if _, err := tmp.WriteString(b.String()); err != nil {
+		tmp.Close()
+		return fmt.Errorf("write %s: %w", path, err)
+	}
+	if err := tmp.Close(); err != nil {
+		return fmt.Errorf("write %s: %w", path, err)
+	}
+	if err := os.Chmod(tmp.Name(), 0o644); err != nil {
+		return err
+	}
+	if err := os.Rename(tmp.Name(), path); err != nil {
+		return fmt.Errorf("replace %s: %w", path, err)
+	}
+	return nil
+}
+
+// formatRule renders a rule as a TOML table, omitting empty fields.
+func formatRule(r Rule) string {
+	var b strings.Builder
+	b.WriteString("[[rule]]\n")
+	write := func(key, value string) {
+		if value != "" {
+			fmt.Fprintf(&b, "%-12s = %s\n", key, strconv.Quote(value))
+		}
+	}
+	write("match", r.Match)
+	write("counterparty", r.Counterparty)
+	write("type", r.Type)
+	write("account", r.Account)
+	write("tag", r.Tag)
+	if r.Transfer {
+		b.WriteString("transfer     = true\n")
+	}
+	return b.String()
+}
+
 // Column locates one field in a CSV row.
 type Column struct {
 	Col       int    `toml:"col"`