Add query execution statistics dashboard

[muk2]

Summary

• Adds a Query Statistics panel accessible via Ctrl+Shift+S showing session and all-time query execution metrics
• Tracks total queries, successful/failed counts, success rate, min/max/avg execution times
• Status bar shows live session query count and average execution time after the first query
• Panel supports scrolling with Up/Down arrows and closes with Esc/q

Implementation Details

src/ui/app.rs

• Added StatsPanel variant to the Focus enum
• Added QueryStats struct with record_query(), avg_time_ms(), success_rate(), and session_avg_time_ms() methods
• Added query_stats and stats_scroll fields to App
• Added handle_stats_input() for panel navigation
• Integrated stats recording into execute_query() flow
• Added Ctrl+Shift+S keyboard shortcut to open stats panel
• 3 unit tests for QueryStats struct

src/ui/components.rs

• Added draw_stats_panel() function rendering a centered overlay with session and all-time statistics
• Updated draw() to render stats panel when focused
• Updated draw_status_bar() to show session query count and average time
• Added Ctrl+Shift+S to help overlay

Test plan

• cargo build compiles without errors
• cargo test - all 228 tests pass
• Manual: Execute queries and verify stats update in status bar
• Manual: Press Ctrl+Shift+S to open stats panel and verify metrics
• Manual: Verify Up/Down scrolling and Esc/q close behavior

Closes #38

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