fix(locale): use count-neutral wording for the experiment days-left message

Address review comments on PR #2778:
- "{{ .Field_1 }} days left" rendered "`1` days left" for an experiment
  with exactly one day remaining. Reword it as "Days left: {{ .Field_1 }}"
  so the English notification is grammatical for singular, plural, zero
  and negative values, and update the test expectation accordingly.

The Japanese message is already count-neutral and stays unchanged.

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

Author: s35560

pkg/locale/localizedata/en.yaml

@@ -157,5 +157,5 @@ AppliedNow: "{{ .Field_1 }} has been applied immediately"
 
 NotificationFeatureStale: "There are feature flags that have not been used for more than {{ .Field_1 }} days."
 NotificationExperimentRunning: "There are running experiments."
-NotificationExperimentDaysLeft: "{{ .Field_1 }} days left"
+NotificationExperimentDaysLeft: "Days left: {{ .Field_1 }}"
 NotificationMAUCount: "This is the MAU for month {{ .Field_1 }}."

pkg/subscription/sender/notifier/slack_test.go

@@ -202,7 +202,7 @@ func TestCreateExperimentRunningAttachment(t *testing.T) {
 			desc:            "english",
 			lang:            locale.En,
 			expectedText:    "There are running experiments.",
-			expectedDaysMsg: "- `1` days left, Name: *",
+			expectedDaysMsg: "- Days left: `1`, Name: *",
 		},
 		{
 			desc:            "japanese",