feat(notifications): optional single summary notification + launcher badge

[j-fuentes]

What

Adds an opt-in mode that groups every new article found during a sync into one notification, plus an optional launcher icon badge with the count.

Today, with several feeds that have "Allow notifications" on, a single background sync can post dozens of notifications (one group per feed + one per article). This keeps that behaviour as the default and adds a quieter alternative.

How

• NotificationHelper buffers new articles while a sync runs (instead of notifying immediately) when the summary mode is on.
• SyncWorker calls notificationHelper.flushSummary() at the end of each sync, which posts a single InboxStyle notification:
  • title: "N new articles" (plural resource)
  • lines: feed · title, up to 8, then "and N more"
  • tapping opens the app
• Launcher badge: setNumber(count) + BADGE_ICON_SMALL on a channel with showBadge = true. Launchers that support numeric badges show the count, the rest show a dot.
• New preferences (DataStore + CompositionLocal, same pattern as the existing ones):
  • notificationSummary — default off, so nothing changes for existing users
  • notificationBadge — default on, only effective when summary is on
• UI: Settings → Usage → new "Notifications" section with both switches.
• Still respects the per-feed "Allow notifications" flag and the account sync interval / Wi-Fi / charging constraints; no new workers.

Notes

• Only English strings added; translations can go through Weblate.
• Built with ./gradlew assembleGithubRelease, unit tests and lint pass (CI on my fork).
• Manually tested on a device: summary notification with count + list after sync, and the launcher badge.

Happy to adjust naming, placement of the settings or defaults.

Disclosure

This code was vibe-coded with an AI assistant. I have reviewed it and tested it manually myself on a device.