Show sync times as minutes and seconds

time_spent and total_time are now "MM:SS" instead of "HH:MM:SS". A sync longer
than an hour keeps counting minutes, e.g. "62:03".

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

internal/backend/sync_test.go

@@ -401,7 +401,10 @@ func TestResetDB(t *testing.T) {
 }
 
 func TestFormatDuration(t *testing.T) {
-	assert.Equal(t, "01:02:03", formatDuration(time.Hour+2*time.Minute+3*time.Second))
+	assert.Equal(t, "00:00", formatDuration(0))
+	assert.Equal(t, "00:07", formatDuration(7*time.Second+900*time.Millisecond), "parts of a second are left out")
+	assert.Equal(t, "02:15", formatDuration(2*time.Minute+15*time.Second))
+	assert.Equal(t, "62:03", formatDuration(time.Hour+2*time.Minute+3*time.Second), "minutes keep counting past an hour")
 }
 
 func TestSyncKeepsAlternativeNamesAndCover(t *testing.T) {