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/alembic/versions/0014_share_status.py

@@ -0,0 +1,67 @@
+"""Add status + accepted_at to heap_shares and folder_shares
+
+Revision ID: 0014_share_status
+Revises: 0013_drop_old_ct
+Create Date: 2026-04-21
+
+Shares used to activate instantly on the owner's side. We now want a
+pending/accepted lifecycle so the recipient gets a notification bell and
+chooses to accept or decline before the shared item shows up in their
+sidebar.
+
+Backfill note: every pre-existing row is treated as `accepted` with
+accepted_at = created_at. This is a pragmatic fiction — it keeps the
+sidebar populated after the migration without anyone having to click
+accept on shares that were already live. Any future "accepted X ago" UI
+inheriting this backfilled timestamp should be aware it's not a real
+user-action moment.
+
+The one-migration trick: we add `status` with `server_default="accepted"`
+so the backfill happens in-place, then drop the default so new inserts
+fall through to the Python-side model default ("pending").
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0014_share_status"
+down_revision: Union[str, None] = "0013_drop_old_ct"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    for table in ("heap_shares", "folder_shares"):
+        op.add_column(
+            table,
+            sa.Column(
+                "status",
+                sa.String(),
+                nullable=False,
+                server_default="accepted",
+            ),
+        )
+        op.add_column(
+            table,
+            sa.Column("accepted_at", sa.DateTime(), nullable=True),
+        )
+        op.execute(
+            f"UPDATE {table} SET accepted_at = created_at "
+            "WHERE accepted_at IS NULL"
+        )
+        # Drop the DB default so new rows inherit the Python-side
+        # model default ("pending") instead of silently auto-accepting.
+        op.alter_column(table, "status", server_default=None)
+        op.create_index(
+            f"ix_{table}_shared_with_status",
+            table,
+            ["shared_with_id", "status"],
+        )
+
+
+def downgrade() -> None:
+    for table in ("heap_shares", "folder_shares"):
+        op.drop_index(f"ix_{table}_shared_with_status", table_name=table)
+        op.drop_column(table, "accepted_at")
+        op.drop_column(table, "status")