feat(sharing): pending-state invites, notification bell, sidebar polish

Shares used to activate instantly on the owner's side with no notice to
the recipient. Introduce a pending/accepted lifecycle so a recipient
gets a bell notification on login and explicitly Accept or Decline
before the shared item lands in their sidebar.

Backend
- Migration 0014 adds `status` + `accepted_at` to heap_shares and
  folder_shares; pre-existing rows are backfilled to 'accepted' so
  nothing disappears from anyone's current sidebar. One-migration trick:
  server_default 'accepted' during add_column, then strip so new inserts
  fall through to the Python model default 'pending'.
- New recipient-only endpoints: POST /sharing/{heaps|folders}/{id}/accept
  (idempotent) and /decline (hard delete, so re-invites are clean).
- New GET /sharing/pending returning {heaps, folders} of outstanding
  invites with target_name + owner_username + permission.
- list_shared_{heaps,folders} now filter to status='accepted' and carry
  share_id so the recipient can Leave without a second lookup.
- ShareResponse exposes status so the owner sees pending invites.

Frontend
- NotificationBell lives in the LeftSidebar user row: a Popover
  triggered by Bell with a count badge. Each row shows owner avatar,
  "{owner} shared {heap|folder} {name}" with a permission subtitle,
  and Accept / Decline inline. Polls /sharing/pending every 60s.
- Shared Avatar helper extracted to sharing/Avatar.tsx — used by
  ShareDialog, NotificationBell, and the sidebar shared rows so one
  user's identity colour is stable everywhere.
- Sidebar shared-row polish: owner avatar bubble + Eye/Pencil
  permission icon (was uppercase pill). Right-click opens a context
  menu with Open / Leave; Leave calls the existing recipient-revoke
  DELETE and invalidates the shared-{heaps,folders} query.
- ShareDialog shows an amber "Invited" pill next to pending recipients.
- New shadcn context-menu primitive (radix dep already installed).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/models/sharing.py

@@ -24,12 +24,19 @@ class HeapShare(Base):
     owner_id = Column(String, ForeignKey("users.id"), nullable=False)
     shared_with_id = Column(String, ForeignKey("users.id"), nullable=False)
     permission = Column(String, nullable=False, default="read")  # 'read' | 'write'
+    # Lifecycle: 'pending' while the recipient hasn't acted, 'accepted'
+    # once they've Accept'd in the notification bell. Decline deletes the
+    # row outright — see migration 0014 for the backfill of pre-existing
+    # rows to 'accepted' so nothing vanishes from existing sidebars.
+    status = Column(String, nullable=False, default="pending")
     created_at = Column(DateTime, server_default=func.now())
+    accepted_at = Column(DateTime, nullable=True)
 
     __table_args__ = (
         UniqueConstraint("heap_id", "shared_with_id", name="uq_heap_share"),
         Index("ix_heap_shares_shared_with", "shared_with_id"),
         Index("ix_heap_shares_heap_id", "heap_id"),
+        Index("ix_heap_shares_shared_with_status", "shared_with_id", "status"),
     )
 
 
@@ -43,10 +50,13 @@ class FolderShare(Base):
     owner_id = Column(String, ForeignKey("users.id"), nullable=False)
     shared_with_id = Column(String, ForeignKey("users.id"), nullable=False)
     permission = Column(String, nullable=False, default="read")  # 'read' | 'write'
+    status = Column(String, nullable=False, default="pending")
     created_at = Column(DateTime, server_default=func.now())
+    accepted_at = Column(DateTime, nullable=True)
 
     __table_args__ = (
         UniqueConstraint("folder_id", "shared_with_id", name="uq_folder_share"),
         Index("ix_folder_shares_shared_with", "shared_with_id"),
         Index("ix_folder_shares_folder_id", "folder_id"),
+        Index("ix_folder_shares_shared_with_status", "shared_with_id", "status"),
     )