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/email.py

@@ -55,6 +55,20 @@ async def is_smtp_configured() -> bool:
     return bool(config.get("smtp_host"))
 
 
+async def get_base_url() -> str:
+    """Get the application base URL from admin settings, falling back to Config.BASE_URL."""
+    async with async_session() as session:
+        result = await session.execute(
+            select(Setting)
+            .join(User, Setting.user_id == User.id)
+            .where(User.role == "admin", Setting.key == "base_url")
+        )
+        setting = result.scalars().first()
+        if setting and setting.value:
+            return setting.value.rstrip("/")
+    return Config.BASE_URL.rstrip("/")
+
+
 async def send_email(to: str, subject: str, html_body: str) -> None:
     """Send an email via SMTP."""
     config = await get_smtp_config()