Add EXPLAIN plan visualization with diagram, tree, and raw views

[datlechin]

Summary

Adds visual EXPLAIN plan rendering for PostgreSQL and MySQL. Users can now see query execution plans as interactive diagrams or tree views instead of raw text.

Three view modes (segmented control)

• Diagram: Canvas-based boxes with arrows, color-coded by cost (green to red), zoom controls, click node for detail popover
• Tree: Native OutlineGroup list with expandable nodes, alternating rows, click to select and see detail panel
• Raw: Original monospace text output (JSON for PostgreSQL/MySQL)

PostgreSQL

• EXPLAIN button dropdown: "EXPLAIN" and "EXPLAIN ANALYZE"
• Uses EXPLAIN (FORMAT JSON) / EXPLAIN (ANALYZE, FORMAT JSON) for structured parsing
• Shows planning time and execution time from ANALYZE

MySQL/MariaDB

• Plain "Explain" button (no dropdown since MariaDB doesn't support EXPLAIN ANALYZE)
• Uses EXPLAIN FORMAT=JSON for structured parsing

Architecture

• QueryPlanNode / QueryPlan data model with cost fraction computation
• QueryPlanParser protocol with PostgreSQL JSON, MySQL JSON, and SQLite parsers
• Databases routed through variant-based EXPLAIN path via explainVariants on DriverPlugin
• Tab state: explainPlan: QueryPlan? on QueryTab, cleared on normal Execute

New files

• TablePro/Models/Query/QueryPlan.swift
• TablePro/Core/Services/Query/QueryPlanParser.swift
• TablePro/Views/QueryPlan/QueryPlanTreeView.swift
• TablePro/Views/QueryPlan/QueryPlanDiagramView.swift

Test plan

• PostgreSQL: Explain dropdown > EXPLAIN > verify diagram with color-coded nodes
• PostgreSQL: Explain dropdown > EXPLAIN ANALYZE > verify actual timing shown
• PostgreSQL: Toggle Diagram / Tree / Raw views
• PostgreSQL: Click node in tree > verify detail panel
• PostgreSQL: Click node in diagram > verify detail popover
• MySQL/MariaDB: Plain Explain button (no dropdown) > verify JSON parsed into tree
• After EXPLAIN, click Execute > verify data grid returns (no stuck explain view)
• Status bar hidden during EXPLAIN view
• ClickHouse: Verify existing multi-variant EXPLAIN still works