Add client-side SQL syntax pre-validation before execution

[muk2]

Summary

Currently, SQL syntax errors are only detected after the query is sent to PostgreSQL. Add lightweight client-side syntax validation using the sqlparser crate (already a dependency) to catch obvious errors before execution, providing faster feedback.

Current Behavior

• User writes a query with a typo (e.g., SELEC * FROM users)
• User presses F5/Ctrl+Enter
• Query is sent to PostgreSQL
• PostgreSQL returns an error
• Error is displayed in the results panel

Proposed Behavior

• When the user presses F5/Ctrl+Enter, the query is first parsed locally with sqlparser
• If parsing fails, show the error immediately in the results panel (or status bar) without sending to PostgreSQL
• If parsing succeeds, send to PostgreSQL as normal
• This should be a pre-execution check only — no real-time/as-you-type validation to avoid being clunky or spammy
• Invalid SQL that sqlparser doesn't catch (e.g., nonexistent tables) still goes to PostgreSQL and gets the existing structured error handling

Design Considerations

• Not a linter or as-you-type checker — only validate on execute to avoid noise
• Fail open — if sqlparser can't parse a query but it might be valid PostgreSQL-specific syntax (e.g., SHOW, COPY, \ commands), skip pre-validation and let PostgreSQL handle it
• Use sqlparser::dialect::PostgreSqlDialect for PostgreSQL-specific syntax support
• Show pre-validation errors using the existing StructuredError display with ErrorCategory::Syntax
• Position/line/column from sqlparser errors should map to the editor cursor position
• Keep the round-trip to PostgreSQL as the authoritative error source — client-side validation is just a fast shortcut for obvious mistakes

Scope

• Parse with sqlparser before sending to PostgreSQL
• Map sqlparser error positions to editor line/column
• Display using existing structured error UI
• Skip pre-validation for statements sqlparser doesn't support (SHOW, COPY, SET, etc.)

Out of Scope

• Real-time/as-you-type syntax highlighting of errors
• Semantic validation (table/column existence)
• Autocomplete integration