Add querychat.data module with sample datasets

Closes #118

Adds a new querychat.data module with built-in sample datasets (titanic() and tips()) to make it easier to get started without external dependencies.

Example

from querychat import QueryChat
from querychat.data import titanic

df = titanic()
qc = QueryChat(df, "titanic")
app = qc.app()

Changes

• New querychat.data module with titanic() and tips() functions
• Datasets stored as gzipped CSVs (8.5KB total) using importlib.resources
• Updated all examples and documentation to use the new data module
• Added comprehensive test suite (10 new tests)