Add invitation system, table of contents, actionable dashboard, and settings improvements

Invitation system (Phase 5.7):
- invitation_tokens table (migration 0012) with SHA256-hashed tokens, 7-day expiry
- Admin CRUD endpoints: POST/GET/DELETE /api/admin/invitations
- Branded invitation email with registration link
- /register-invite frontend view with token validation and account creation
- Admin UI: invite form + pending invitations table with revoke
- Configurable base URL setting for email links (replaces hardcoded Config.BASE_URL)

Table of contents + markdown improvements (Phase 5.8):
- TableOfContents component: sticky sidebar, heading parsing, smooth-scroll
- Custom marked renderer adds id attributes to headings for anchor links
- stripFirstLineTags() prevents leading #tags from rendering as headings
- NoteViewerView/TaskViewerView flex layout with TOC sidebar (hidden ≤1200px)
- Model catalog refresh: added llama3.2/3.3, gemma3, qwen3, phi4, deepseek-r1,
  qwen2.5-coder, dolphin3; added Reasoning category; removed discontinued models
- Settings model section split into Installed/Available tabs

Actionable dashboard (Phase 5.9):
- due_before/due_after query params on /api/tasks (exclusive < / inclusive >=)
- HomeView rewritten: overdue (red accent), due today, in progress, chats, notes
- 5 parallel API calls via Promise.allSettled
- Client-side done filtering and in-progress deduplication
- Task sections hidden when empty; status toggle removes done tasks

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

src/fabledassistant/services/notifications.py

@@ -138,6 +138,33 @@ async def send_password_reset_success_email(email: str) -> None:
     await send_email(email, "Fabled Assistant - Password Changed", html)
 
 
+async def send_invitation_email(email: str, invite_url: str, invited_by_username: str) -> None:
+    """Send a branded invitation email with a registration link."""
+    html = f"""
+    <div style="font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif; max-width: 480px; margin: 0 auto; padding: 24px;">
+        <div style="background: #6366f1; color: #fff; padding: 16px 24px; border-radius: 8px 8px 0 0; text-align: center;">
+            <h1 style="margin: 0; font-size: 20px;">You're Invited!</h1>
+        </div>
+        <div style="background: #f9fafb; padding: 24px; border: 1px solid #e5e7eb; border-top: none; border-radius: 0 0 8px 8px;">
+            <p style="margin: 0 0 16px; color: #374151; font-size: 15px;">
+                <strong>{invited_by_username}</strong> has invited you to join Fabled Assistant. Click the button below to create your account:
+            </p>
+            <div style="text-align: center; margin: 24px 0;">
+                <a href="{invite_url}" style="display: inline-block; background: #6366f1; color: #fff; padding: 12px 32px; border-radius: 6px; text-decoration: none; font-weight: 600; font-size: 15px;">
+                    Accept Invitation
+                </a>
+            </div>
+            <p style="margin: 0 0 8px; color: #6b7280; font-size: 13px;">This invitation expires in 7 days.</p>
+            <p style="margin: 0; color: #6b7280; font-size: 13px;">If you weren't expecting this invitation, you can safely ignore this email.</p>
+            <hr style="border: none; border-top: 1px solid #e5e7eb; margin: 16px 0;" />
+            <p style="margin: 0; color: #9ca3af; font-size: 12px;">If the button doesn't work, copy and paste this link into your browser:</p>
+            <p style="margin: 4px 0 0; color: #9ca3af; font-size: 12px; word-break: break-all;">{invite_url}</p>
+        </div>
+    </div>
+    """
+    await send_email(email, "Fabled Assistant - You're Invited!", html)
+
+
 async def check_due_tasks() -> None:
     """Check for tasks due today and send reminder emails."""
     if not await is_smtp_configured():