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>

frontend/src/components/sharing/ShareDialog.tsx

@@ -2,6 +2,7 @@ import { useEffect, useMemo, useState } from 'react'
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
 import { Users, Eye, Pencil, X, Loader2 } from 'lucide-react'
 import { cn } from '@/lib/utils'
+import { Avatar } from './Avatar'
 import {
   sharing,
   type ShareInfo,
@@ -260,8 +261,18 @@ export function ShareDialog({
                 >
                   <Avatar name={share.shared_with_username} />
                   <div className="min-w-0 flex-1">
-                    <div className="truncate text-sm font-medium text-text">
-                      {share.shared_with_username}
+                    <div className="flex items-center gap-1.5">
+                      <span className="truncate text-sm font-medium text-text">
+                        {share.shared_with_username}
+                      </span>
+                      {share.status === 'pending' && (
+                        <span
+                          className="rounded-sm bg-amber-500/15 px-1.5 py-0.5 text-[10px] font-semibold uppercase tracking-wide text-amber-300"
+                          title="Waiting for them to accept"
+                        >
+                          Invited
+                        </span>
+                      )}
                     </div>
                     <div className="flex items-center gap-1 text-[11px] text-text-muted">
                       {share.permission === 'write' ? (
@@ -303,35 +314,3 @@ export function ShareDialog({
   )
 }
 
-// ── Helpers ────────────────────────────────────────────────────────────
-
-/** Circular initial bubble. Hashes the username into one of a small
- *  set of stable palette tints so each person's avatar reads
- *  consistently across the app — the colour isn't meaningful, just an
- *  identity cue that makes a list of names feel less anonymous. */
-function Avatar({ name, size = 'md' }: { name: string; size?: 'sm' | 'md' }) {
-  const initials = name.slice(0, 2).toUpperCase()
-  const palette = [
-    'bg-primary/25 text-primary',
-    'bg-pick/25 text-pick',
-    'bg-blue-500/25 text-blue-300',
-    'bg-purple-500/25 text-purple-300',
-    'bg-amber-500/25 text-amber-300',
-  ]
-  let hash = 0
-  for (let i = 0; i < name.length; i++) hash = (hash * 31 + name.charCodeAt(i)) | 0
-  const tint = palette[Math.abs(hash) % palette.length]
-  const dims = size === 'sm' ? 'h-5 w-5 text-[9px]' : 'h-8 w-8 text-[11px]'
-  return (
-    <span
-      className={cn(
-        'inline-flex shrink-0 items-center justify-center rounded-full font-semibold uppercase tracking-wide',
-        dims,
-        tint
-      )}
-      aria-hidden
-    >
-      {initials}
-    </span>
-  )
-}