feat: mouse support for editor and UI navigation

[muk2]

Summary

Adds mouse support to pgrsql, addressing #20. Mouse interactions complement keyboard navigation.

• Click to position cursor in editor
• Click to select sidebar items and switch focus to sidebar/editor/results
• Click to select rows in results table
• Scroll wheel works in editor, sidebar, and results
• Layout tracking — LayoutRects stored during each draw for coordinate mapping
• Mouse events ignored during connection dialog and help overlay

Mouse Actions

Area	Click	Scroll
Editor	Position cursor at click location	Scroll editor content
Sidebar	Select item at click position	Navigate list up/down
Results	Select row at click position	Scroll results up/down

Changes

File	Change
src/ui/app.rs	Added LayoutRects struct, handle_mouse(), handle_mouse_click(), handle_mouse_scroll(), rect_contains() helper
src/ui/components.rs	draw() now takes &mut App, stores panel rects to app.layout
src/main.rs	Added Event::Mouse handling in event loop

Test plan

• All 124 existing tests pass
• Clippy clean
• cargo fmt clean
• Verify clicking in editor positions cursor
• Verify clicking sidebar items selects them
• Verify clicking in results selects the row
• Verify scroll wheel works in all three panes
• Verify keyboard navigation still works identically
• Verify mouse is ignored during connection dialog

Closes #20

🤖 Generated with Claude Code

[muk2]

Cursor clicking kind of works. Does not work to place cursor in editor. weirdly selects/unselects on schema/tables. works but not as smooth and user intuitive. requires extra UI polishing