fix(06.2): resolve four sharing UX issues found in re-test UAT

- AccountView: remove hardcoded @ prefix so handle matches what share dialog expects
- documents store: set is_shared=true optimistically after successful share so badge shows without refetch
- GET /api/documents/{id}: allow recipients of an active share to view the document (was returning 404 for non-owners)
- ShareModal: move Share button to its own full-width row so it no longer overflows the input area

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/AccountView.vue

@@ -9,7 +9,7 @@
         <h3 class="font-semibold text-gray-800 mb-4">Account information</h3>
         <div class="space-y-2 text-sm text-gray-700">
           <div><span class="text-gray-500">Email:</span> {{ authStore.user?.email }}</div>
-          <div><span class="text-gray-500">Username:</span> @{{ authStore.user?.handle }}</div>
+          <div><span class="text-gray-500">Username:</span> {{ authStore.user?.handle }}</div>
           <div class="flex items-center gap-2">
             <span class="text-gray-500">Role:</span>
             <span