Add EXPLAIN ANALYZE visual query plan viewer

[muk2]

Summary

Implements Issue #12 — a visual query plan viewer that parses EXPLAIN output into a color-coded tree.

• Automatically detects EXPLAIN/EXPLAIN ANALYZE queries
• Renders the execution plan as a visual tree with cost annotations
• Color-coded nodes: green (fast, <10% total), yellow (moderate, 10-30%), red (slow, >30%)
• Shows estimated vs actual row count mismatches with warning indicators
• Displays planning time and execution time footer
• Ctrl+E toggles between visual tree and raw text views

Changes

File	Changes
src/explain.rs	New module: PlanNode, QueryPlan structs, EXPLAIN text parser, color classification, 9 unit tests
src/main.rs	Added mod explain
src/ui/app.rs	Added explain_plans, show_visual_plan, plan_scroll fields, EXPLAIN detection in execute_query(), Ctrl+E toggle in results handler
src/ui/components.rs	Added draw_explain_plan(), render_plan_node() tree renderer with color-coded spans, updated help overlay

Test plan

• All 133 tests pass (124 existing + 9 new)
• cargo clippy clean
• cargo fmt clean
• Manual test: Run EXPLAIN SELECT * FROM table → visual plan displays
• Manual test: Run EXPLAIN ANALYZE SELECT ... → plan with actual times
• Manual test: Ctrl+E toggles between visual and raw view
• Manual test: Color coding reflects relative cost of each node
• Manual test: Row estimate mismatches shown with warning

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