Add direct pane navigation shortcuts and error copy support

[muk2]

Summary

• Added Alt+1/2/3 shortcuts to jump directly to Sidebar/Editor/Results from any non-dialog context, eliminating the need to cycle through panes with Tab
• Enhanced Ctrl+C in the Results pane to copy error messages (including detail, hint, and location info) when an error result is displayed, making it easy to share errors for debugging
• Updated the help overlay to document the new shortcuts

What Changed

• src/ui/app.rs: Added Alt+1/2/3 handling in global input handler; enhanced copy_selected_cell() to detect and copy error messages with full context (category, message, detail, hint, line/column)
• src/ui/components.rs: Updated help overlay text to include Alt+1/2/3 navigation and clarified Ctrl+C copies both cell values and error messages

Test Plan

• Press Alt+1 from the Editor — should jump to Sidebar
• Press Alt+2 from the Sidebar — should jump to Editor
• Press Alt+3 from the Editor — should jump to Results
• Alt+1/2/3 should NOT work during Connection Dialog, Help overlay, or Export Picker
• Execute a query that produces an error, then press Ctrl+C in Results — should see "Error message copied to clipboard" toast and the clipboard should contain the formatted error
• Execute a successful query, navigate to a cell, press Ctrl+C — should still copy the cell value as before
• Open help (?) — should show the new Alt+1/2/3 shortcut line
• Run cargo test — all 230 tests pass

Closes #56

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