Add LLM tool calling for creating tasks, notes, and searching from chat

Ollama tool/function calling integration allows the LLM to create tasks,
create notes, and search existing notes on behalf of the user during chat.
Multi-round tool loop (max 5 rounds) lets the model execute tools then
produce a natural language response. Tool results are persisted in a new
JSONB column on messages and rendered as compact cards with linked titles.

- Migration 0013: add tool_calls JSONB column to messages
- New services/tools.py: tool definitions + execute_tool dispatcher
- llm.py: ChatChunk dataclass, stream_chat_with_tools(), date in system prompt
- generation_task.py: multi-round tool call loop with SSE tool_call events
- Frontend: ToolCallRecord type, streamingToolCalls in store, ToolCallCard
  component, rendering in ChatMessage and ChatView streaming bubble

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

alembic/versions/0013_add_tool_calls_to_messages.py

@@ -0,0 +1,17 @@
+"""Add tool_calls JSONB column to messages."""
+
+from alembic import op
+
+revision = "0013"
+down_revision = "0012"
+
+
+def upgrade() -> None:
+    op.execute("""
+        ALTER TABLE messages
+        ADD COLUMN IF NOT EXISTS tool_calls JSONB
+    """)
+
+
+def downgrade() -> None:
+    op.drop_column("messages", "tool_calls")