Add query execution statistics dashboard

[muk2]

Summary

Adds a query execution statistics dashboard that tracks and displays performance metrics across the session.

• Stats panel: Ctrl+Shift+S opens a centered overlay showing session and all-time statistics
• Session metrics: Queries executed, total time, average time
• All-time metrics: Total queries, success/failure count, success rate, min/max/avg execution time
• Status bar: Shows live session stats (query count + avg time) in the bottom status bar
• Scrollable: Panel supports scrolling for smaller terminals

Metrics Displayed

Metric	Description
Queries executed	Session and all-time count
Success rate	Percentage of successful queries
Average time	Mean execution time
Min/Max time	Fastest and slowest queries
Session total	Cumulative execution time

Files Changed

File	Description
src/ui/app.rs	QueryStats struct, state tracking, input handler, recording
src/ui/components.rs	Stats panel rendering, status bar session stats

Test plan

• cargo build passes
• cargo test passes (3 new unit tests for QueryStats)
• Ctrl+Shift+S opens stats panel
• Stats update after each query execution
• Status bar shows session stats after first query
• Esc closes stats panel

Closes #38

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