Add SQL query formatter/prettifier (Ctrl+Shift+F)

[muk2]

Summary

Implements #33 — adds a built-in SQL query formatter that auto-formats messy or one-line SQL into well-indented, readable SQL directly in the editor.

• New ast::formatter module with format_sql() function that pretty-prints Query AST nodes
• Shortcut: Ctrl+Shift+F in the editor formats the entire buffer
• Multi-line formatting: Each major SQL clause on its own line (SELECT, FROM, WHERE, JOIN, GROUP BY, HAVING, ORDER BY, LIMIT, OFFSET)
• Indented projections: Multi-column SELECT lists get one column per line with proper indentation
• JOIN formatting: JOIN conditions indented under their JOIN clause
• All query types: SELECT, INSERT, UPDATE, DELETE, CTEs, UNION/INTERSECT/EXCEPT
• Non-destructive: Shows toast error if SQL can't be parsed; buffer unchanged
• Single undo step: Ctrl+Z reverts the entire format operation
• 16 new unit tests including round-trip reparse verification ensuring formatted SQL is always valid

Example

Before (single line):

SELECT u.id, u.name, o.total FROM users u JOIN orders o ON u.id = o.user_id WHERE o.total > 100 ORDER BY o.total DESC LIMIT 10

After (Ctrl+Shift+F):

SELECT
    u.id,
    u.name,
    o.total
FROM users AS u
JOIN orders AS o
    ON u.id = o.user_id
WHERE o.total > 100
ORDER BY o.total DESC
LIMIT 10

Test plan

• cargo build compiles without warnings
• cargo test — all 232 tests pass (16 new formatter tests)
• Formatter produces valid SQL (round-trip reparse test)
• Multi-column SELECT indentation
• JOIN/WHERE/GROUP BY/ORDER BY each on own line
• CTE formatting
• INSERT/UPDATE/DELETE formatting
• Help overlay shows new shortcut

Closes #33

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