feat: editable SQL query editor in $app()

[gadenbuie]

Closes #70

Summary

Replaces the read-only SQL display in $app() with an interactive bslib::input_code_editor() (R) and its Python equivalent, allowing users to directly edit and run SQL queries from the UI.

Key design decisions:

• Submit semantics: Relies on the editor's built-in trigger (blur or Ctrl/Cmd+Enter) — no explicit "Run Query" button needed.
• Sync without re-render: A dedicated sync_sql_editor observer calls update_code_editor() on every sql_val() change (LLM update, reset, table switch) to keep the editor current without re-rendering the widget.
• Reset feedback-loop fix: on_sql_editor compares incoming SQL to SELECT * FROM <table> and writes NULL on a match, so the Reset button correctly disappears after a reset even though update_code_editor() triggers the observer.
• No new dependencies: bslib::input_code_editor() / update_code_editor() were already available from the viz feature.
• sql_editor is excluded from Shiny bookmarking.

Both R (pkg-r/R/QueryChat.R) and Python (pkg-py/src/querychat/_shiny.py) updated in parallel.

Verification

querychat_app(penguins)

1. Ask the LLM a question — the SQL panel should now be an editable code editor showing the generated query.
2. Manually edit the SQL and press Ctrl/Cmd+Enter (or click away) — the table updates immediately.
3. Click Reset — the editor reverts to SELECT * FROM penguins and the Reset button disappears.
4. For multi-table setups, switch the active table — the editor should update to show the correct query.