Introduce internal/ui package and rewrite user-facing output

All user-facing output now goes through a single ui.Writer with a
uniform style:

  》 (white)     for begin / info / notice
  》 (green)     for complete / success
  Warning:      for warnings (orange)
  ERROR:        for errors (red)
    》          (indented) for progress heartbeats

Color is enabled when stdout is a TTY and NO_COLOR is unset.

Standards:
- Complete-sentence messages with fully qualified terms ("backup
  destination store", "local index database", "snapshot source
  files enumeration").
- Every Complete has a matching Begin.
- Natural verb tense conveys state ("Uploading" -> "Uploaded"). The
  words "begin"/"complete" never appear in message bodies; the marker
  color carries that information.
- ETA means clock time, not duration. Progress lines say "estimated
  remaining time (<dur>), finish at <time>" with both labeled.

Adds globals.CommitDate (populated by Makefile/Dockerfile/goreleaser
via ldflags from `git show -s --format=%cI HEAD`) and a startup banner
printed once per invocation.

Strips fx call-chain noise from startup errors so users see the actual
underlying error (e.g. "creating base path: mkdir /Volumes/BACKUPS:
permission denied" instead of three layers of "could not build
arguments for function ...").

README documents the output style and the ui package conventions.

internal/vaultik/helpers.go

@@ -17,37 +17,6 @@ type SnapshotInfo struct {
 	CompressedSize int64            `json:"compressed_size"`
 }
 
-// formatNumber formats a number with commas
-func formatNumber(n int) string {
-	str := fmt.Sprintf("%d", n)
-	var result []string
-	for i, digit := range str {
-		if i > 0 && (len(str)-i)%3 == 0 {
-			result = append(result, ",")
-		}
-		result = append(result, string(digit))
-	}
-	return strings.Join(result, "")
-}
-
-// formatDuration formats a duration in a human-readable way
-func formatDuration(d time.Duration) string {
-	if d < time.Second {
-		return fmt.Sprintf("%dms", d.Milliseconds())
-	}
-	if d < time.Minute {
-		return fmt.Sprintf("%.1fs", d.Seconds())
-	}
-	if d < time.Hour {
-		mins := int(d.Minutes())
-		secs := int(d.Seconds()) % 60
-		return fmt.Sprintf("%dm %ds", mins, secs)
-	}
-	hours := int(d.Hours())
-	mins := int(d.Minutes()) % 60
-	return fmt.Sprintf("%dh %dm", hours, mins)
-}
-
 // formatBytes formats bytes in a human-readable format
 func formatBytes(bytes int64) string {
 	const unit = 1024