Add structured SQL error reporting with rich context

[muk2]

Summary

• Replaces generic Option<String> error field with StructuredError type that extracts rich information from PostgreSQL DbError responses
• Categorizes errors into Syntax, Semantic, Execution, Transaction, Connection types based on SQLSTATE codes
• Computes line/column numbers from PostgreSQL's byte-position error offset
• Renders color-coded, multi-line error display in results panel with category labels, SQLSTATE codes, detail, hints, and object context

Implementation Details

src/db/query.rs

• Added ErrorCategory enum with Syntax, Semantic, Execution, Transaction, Connection, Unknown variants
• Added StructuredError struct with fields: category, severity, code, message, detail, hint, position, line, col, schema, table, column, constraint, where_
• Added from_pg_error() to extract rich info from tokio_postgres::Error via as_db_error() API
• Added byte_offset_to_line_col() to convert PostgreSQL's 1-based byte position to (line, column)
• Added categorize_sqlstate() mapping SQLSTATE class codes to error categories
• Changed QueryResult.error from Option<String> to Option<StructuredError>
• 5 new unit tests for structured error types, display, position conversion, and SQLSTATE categorization

src/ui/components.rs

• Added draw_structured_error() function rendering rich error display with:
  • Color-coded category header (red for syntax/execution, yellow for semantic/transaction)
  • SQLSTATE code badge
  • Line/column position info highlighted in accent color
  • Detail and Hint sections with bold labels
  • Schema/table/column object context
  • Constraint name
  • PL/pgSQL call stack context
• Results panel title now shows specific error category instead of generic "ERROR"

src/ui/app.rs

• Status bar error messages now show category label (e.g., "Syntax Error: ...")

Error Category Mapping

SQLSTATE Class	Category	Example
42601	Syntax Error	syntax_error
42P01, 42703	Semantic Error	undefined_table, undefined_column
22, 23	Execution Error	division_by_zero, constraint_violation
25, 40	Transaction Error	transaction_aborted
08	Connection Error	connection_exception

Test plan

• cargo build compiles without warnings
• cargo test - all 230 tests pass
• Manual: Execute invalid SQL and verify structured error with line/column
• Manual: Reference non-existent table and verify "Semantic Error" category
• Manual: Trigger constraint violation and verify "Execution Error" with constraint name
• Manual: Verify hints are displayed when PostgreSQL provides them

Closes #44

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