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/ui/ui_test.go

@@ -0,0 +1,86 @@
+package ui
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+	"time"
+)
+
+func newTestWriter(color bool) (*Writer, *bytes.Buffer) {
+	buf := &bytes.Buffer{}
+	return NewWithColor(buf, color), buf
+}
+
+func TestMessageMethodsPlain(t *testing.T) {
+	tests := []struct {
+		method string
+		fn     func(*Writer)
+		want   string
+	}{
+		{"Begin", func(w *Writer) { w.Begin("starting %s", "thing") }, "》 starting thing\n"},
+		{"Complete", func(w *Writer) { w.Complete("done %s", "thing") }, "》 done thing\n"},
+		{"Info", func(w *Writer) { w.Info("status") }, "》 status\n"},
+		{"Notice", func(w *Writer) { w.Notice("note") }, "》 note\n"},
+		{"Warning", func(w *Writer) { w.Warning("oops") }, "Warning: oops\n"},
+		{"Error", func(w *Writer) { w.Error("boom") }, "ERROR: boom\n"},
+		{"Progress", func(w *Writer) { w.Progress("p") }, "  》 p\n"},
+		{"Banner", func(w *Writer) { w.Banner("hello") }, "hello\n"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.method, func(t *testing.T) {
+			w, buf := newTestWriter(false)
+			tt.fn(w)
+			if got := buf.String(); got != tt.want {
+				t.Errorf("got %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestColorOutputContainsANSI(t *testing.T) {
+	w, buf := newTestWriter(true)
+	w.Error("boom")
+	out := buf.String()
+	if !strings.Contains(out, "\033[") {
+		t.Errorf("expected ANSI escapes in color output, got %q", out)
+	}
+	if !strings.Contains(out, "ERROR: ") {
+		t.Errorf("expected 'ERROR: ' text in output, got %q", out)
+	}
+}
+
+func TestValueFormattersPlain(t *testing.T) {
+	w, _ := newTestWriter(false)
+
+	if got := w.Hex("0123456789abcdef0123"); got != "0123456789ab..." {
+		t.Errorf("Hex long: got %q", got)
+	}
+	if got := w.Hex("short"); got != "short" {
+		t.Errorf("Hex short: got %q", got)
+	}
+	if got := w.Size(1024); got != "1.0 kB" {
+		t.Errorf("Size: got %q", got)
+	}
+	if got := w.Duration(90 * time.Second); got != "1m30s" {
+		t.Errorf("Duration: got %q", got)
+	}
+	if got := w.Count(12345); got != "12,345" {
+		t.Errorf("Count: got %q", got)
+	}
+	if got := w.Percent(12.34); got != "12.3%" {
+		t.Errorf("Percent: got %q", got)
+	}
+}
+
+func TestValueFormattersColored(t *testing.T) {
+	w, _ := newTestWriter(true)
+	hex := w.Hex("0123456789abcdef0123")
+	if !strings.Contains(hex, "\033[") {
+		t.Errorf("expected ANSI in colored Hex output, got %q", hex)
+	}
+	if !strings.Contains(hex, "0123456789ab") {
+		t.Errorf("expected hex content in output, got %q", hex)
+	}
+}