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/models/__init__.py

@@ -17,3 +17,4 @@ from fabledassistant.models.setting import Setting  # noqa: E402, F401
 from fabledassistant.models.user import User  # noqa: E402, F401
 from fabledassistant.models.app_log import AppLog  # noqa: E402, F401
 from fabledassistant.models.password_reset import PasswordResetToken  # noqa: E402, F401
+from fabledassistant.models.invitation import InvitationToken  # noqa: E402, F401

src/fabledassistant/models/invitation.py

@@ -0,0 +1,25 @@
+from datetime import datetime, timezone
+
+from sqlalchemy import Boolean, DateTime, ForeignKey, Index, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from fabledassistant.models import Base
+
+
+class InvitationToken(Base):
+    __tablename__ = "invitation_tokens"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    email: Mapped[str] = mapped_column(Text, nullable=False)
+    token_hash: Mapped[str] = mapped_column(Text, nullable=False, unique=True)
+    invited_by: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+    expires_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+    used: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+
+    __table_args__ = (
+        Index("ix_invitation_tokens_token_hash", "token_hash"),
+        Index("ix_invitation_tokens_email", "email"),
+    )