feat(notifications): add 'Clear all' button to notification panel (fixes #22)

[aagear]

Description

Fixes #22

Adds a "Clear all" button to the notification panel dropdown next to the existing "Mark all read" button.

Changes

1. frontend/lib/notifications.ts — New clearAllNotifications(supabase, userId) function:

   • Deletes ALL notifications for the user via the existing RLS DELETE policy
   • Best-effort (console.warn on error, same pattern as other notification writers)

2. frontend/components/NotificationsBell.tsx — "Clear all" button in the panel header:

   • Button visible only when items.length > 0
   • Two-step confirmation: click "Clear all" → confirmation prompt ("Delete all? Yes / No") → destructive delete
   • Optimistic UI: the list empties immediately, the DB delete runs in the background
   • Consistent styling with the existing "Mark all read" button pattern

Verification

• Button hidden when inbox is empty
• Confirmation prompt before deletion
• Optimistic UI: list clears immediately on confirm
• No new dependencies

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