fix(strings): reconcile EN/AR parity for default_notification_channel_id (E01.S06)

[abdout]

Story: E01.S06 (Phase 1 Pilot v1, Sprint P1)
Epic: EPIC-PROD-01 Foundation Hardening
Points: 1

Problem

app/build.gradle.kts:111 checkStringParity task fails because app/src/main/res/values/strings.xml has 47 keys while values-ar/strings.xml has 46. Diff identifies one EN-only key:

• default_notification_channel_id = "default" (line 29)

This is a programmatic identifier passed to NotificationChannel(id, …), not a user-facing string — same value must apply in both locales.

Fix

Add the matching entry to values-ar/strings.xml with the identical value default (the channel ID must not be translated; AR users still look up default programmatically). Add an XML comment explaining intent so future translators don't try to localise it.

Acceptance criteria

• :app:checkStringParity passes (EN count == AR count for app module)
• Both files contain default_notification_channel_id with value default
• Comment in both files documents that this is a programmatic identifier

Source: .bmad/epics/epic-prod-01-foundation-hardening.md
Manifest: .bmad/epics/phase-1-pilot-v1.md