feat: extract import into plugin system

[datlechin]

Summary

• Extract SQL import from hardcoded ImportService into the plugin architecture, mirroring the export plugin system exactly
• Add 5 new protocols/types in TableProPluginKit: ImportFormatPlugin, PluginImportDataSink, PluginImportSource, PluginImportProgress, and import result/error types
• Create SQLImportPlugin as the first .tableplugin import bundle (SQL files + .gz)
• Replace ImportDialog with plugin-aware UI: format picker, plugin-provided options, dynamic file type filtering
• Extract ProgressUpdateCoalescer from ExportService as shared utility for both export and import

Test plan

• Open Import dialog — SQL format shown, plugin options (transaction wrap + FK checks) visible
• Select .sql file — preview renders, statement count updates
• Select .sql.gz file — decompression + preview works
• Click Import — progress dialog shows, statements execute, success dialog appears
• Cancel mid-import — rollback executes, error view shows cancellation
• Verify query history records import (success and failure cases)
• Disable SQLImportPlugin in Settings > Plugins — verify Import dialog shows no formats
• Export still works unchanged (CSV, JSON, SQL, XLSX, MQL formats)

[chatgpt-codex-connector]

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