Refactor: improve code quality, maintainability, and accessibility

[mikenikles]

Summary

This PR implements a comprehensive code quality improvement plan focusing on better maintainability, type safety, and accessibility.

Phase 1: Error Handling Framework

• Created centralized error handling in src/lib/errors/
• Added Result type for explicit error handling
• Added AppError interface with error codes and severity levels
• Added handleError function with toast/console routing based on severity

Phase 2: Accessibility Improvements

• Added aria-labels to pagination buttons in query-editor
• Added aria-labels and keyboard handlers (onkeydown for Enter key) to query-history items
• Added aria-labels to sidebar-left star/delete buttons
• Added translations for all accessibility labels in all 6 languages

Phase 3: Type Organization

• Reorganized types from single types.ts into src/lib/types/ directory:
  • database.ts - DatabaseConnection, DatabaseType, SSHTunnelConfig
  • schema.ts - SchemaTable, SchemaColumn, SchemaIndex, SchemaTab
  • query.ts - QueryTab, QueryResult, StatementResult, QueryHistoryItem, SavedQuery
  • explain.ts - ExplainTab, ExplainPlanNode, ExplainResult
  • erd.ts - ErdTab
  • persisted.ts - All PersistedXxx types
• Added JSDoc documentation to all exported interfaces
• Created barrel export maintaining backward compatibility

Phase 4: State Management Migration (Map → Record)

• Migrated all connection-keyed state from Map<string, T> to Record<string, T>
• Updated all 11 manager files to use spread syntax instead of Map operations
• Created record-utils.ts with helper functions
• Deleted map-utils.ts

Benefits:

• Simpler state updates using spread syntax: {...state, [key]: value}
• Better Svelte 5 reactivity (no need for full Map replacement)
• More idiomatic TypeScript/JavaScript patterns

Phase 5-6: Component Extraction

• Split query-editor.svelte from 703 → 400 lines into subcomponents:
  • query-toolbar.svelte - Execute, format, save buttons
  • query-result-tabs.svelte - Multi-statement result tabs
  • query-export-menu.svelte - Export/copy dropdown
  • query-pagination.svelte - Page navigation controls
  • query-error-display.svelte - Error display for failed statements
• Extracted ssh-tunnel-config.svelte from connection-dialog.svelte

Test plan

• Run npm run check - TypeScript must pass ✅
• Run npm run tauri dev - App must start
• Test query execution with multiple statements
• Test pagination controls
• Test export/copy functionality
• Test SSH tunnel connection
• Verify accessibility with keyboard navigation
• Test in all supported languages

🤖 Generated with Claude Code