[Feature]: Notification badge for pending Suggestions

[bjrump]

Problem Statement

Pending Suggestions are only discoverable by navigating to Settings → Suggestions. Action feedback is shown via ephemeral toasts, so a reviewer has no persistent cue that new Suggestions are waiting.

Proposed Solution

• API: a lightweight count endpoint returning the number of pending Suggestions across the Projects the current user may manage.
• Frontend: a badge on the settings entry point / Suggestions Section showing the count; refresh by polling (e.g. every 30s). The sidebar already has an "unread" concept for chats (apps/frontend/components/sidebar/AppSidebar.tsx) — reuse that visual pattern for consistency.

Acceptance Criteria

• A numeric badge reflects the count of pending Suggestions the user may review.
• Count updates without a full reload (polling or invalidation after Apply/Dismiss).
• Badge hidden when count is 0 and for users with no Suggestion-management rights.
• de + en strings.

Scope / Notes

Frontend + small backend (count endpoint). Keep it scoped to Suggestions for now; a general notification inbox is a larger, separate effort.