feat: unified sessions — merge assistant chat into ai_sessions table

Add session_type ('guided'|'chat') and title columns to ai_sessions,
enabling both FlowPilot guided sessions and assistant chat sessions to
live in a single table. This is the foundation for a unified session
history and consistent UX across both interaction modes.

Backend:
- Migration 066: session_type + title columns
- unified_chat_service: chat sessions on ai_sessions with same AI/RAG
- POST /ai-sessions supports session_type='chat' creation
- POST /ai-sessions/{id}/chat for chat messages
- DELETE /ai-sessions/{id} for session deletion
- session_type filter on GET /ai-sessions

Frontend:
- AssistantChatPage rewired to aiSessionsApi (no more assistantChatApi)
- /assistant/:sessionId route for deep-linking
- Session history: type filter pills (All/Guided/Chat), type icons
- Dashboard: both types shown with correct routing and icons
- Fixed glass-border → border-default in dashboard components

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/alembic/versions/066_add_session_type_and_title_to_ai_sessions.py

@@ -0,0 +1,42 @@
+"""Add session_type and title columns to ai_sessions for unified sessions.
+
+Revision ID: 066
+Revises: 065
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "066"
+down_revision = "065"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "ai_sessions",
+        sa.Column(
+            "session_type",
+            sa.String(10),
+            nullable=False,
+            server_default="guided",
+            comment="Session type: guided (FlowPilot) or chat (assistant)",
+        ),
+    )
+    op.add_column(
+        "ai_sessions",
+        sa.Column(
+            "title",
+            sa.String(255),
+            nullable=True,
+            comment="Display title for chat sessions; guided sessions use problem_summary",
+        ),
+    )
+    op.create_index("ix_ai_sessions_session_type", "ai_sessions", ["session_type"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_ai_sessions_session_type", table_name="ai_sessions")
+    op.drop_column("ai_sessions", "title")
+    op.drop_column("ai_sessions", "session_type")