Add client-side SQL syntax pre-validation before execution

[muk2]

Summary

• Adds lightweight client-side SQL syntax validation using the sqlparser crate (already a dependency) that runs before sending queries to PostgreSQL
• Catches obvious syntax errors (typos like SELEC, missing clauses, etc.) instantly without a database round-trip
• Skips pre-validation for PostgreSQL-specific commands that sqlparser doesn't support (SHOW, SET, COPY, VACUUM, DO, etc.) — these are sent directly to PostgreSQL
• Extracts line/column information from sqlparser error messages and displays them using the existing structured error UI
• Fails open: if sqlparser rejects a query that might be valid PostgreSQL-specific syntax, a hint is shown suggesting the parser may not support it

What Changed

• src/ui/app.rs: Added prevalidate_sql() method and parse_line_col_from_sqlparser_error() helper. Integrated pre-validation into the execute_query() flow — if parsing fails, the error is shown immediately without contacting the database.

Test Plan

• Write an obviously invalid query (e.g., SELEC * FROM users) and press F5 — should see an instant syntax error without hitting the database
• Write a valid query (e.g., SELECT * FROM users LIMIT 10) — should execute normally against the database
• Write a PostgreSQL-specific command (e.g., SHOW server_version) — should skip pre-validation and execute on the server
• Write a query with a \ meta-command prefix — should skip pre-validation
• Verify the error display shows line/column information when available
• Run cargo test — all 230 tests pass

Closes #55

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