feat: undo/redo for the SQL editor

[muk2]

Summary

Implements full undo/redo functionality for the SQL editor, addressing #15.

• Ctrl+Z — undo last edit
• Ctrl+Shift+Z or Ctrl+Y — redo
• Intelligent grouping — consecutive character inserts are grouped into a single undo unit, consecutive deletes are grouped, while newlines/paste/cut create separate undo points
• Cursor position restored on undo/redo
• Max 500 undo operations (configurable)
• set_text() and clear() reset undo history (loading from history/clearing editor starts fresh)

Edit Grouping Rules

Action	Grouping
Typing characters	Grouped until a different action type
Newline (Enter)	New undo unit
Backspace/Delete	Grouped together
Paste	Single undo unit
Cut	Single undo unit
Delete selection	Single undo unit

Changes

File	Change
src/editor/buffer.rs	Added UndoHistory, BufferSnapshot, UndoActionType; save_undo(), undo(), redo() methods; integrated undo recording into all mutation methods
src/ui/app.rs	Wired Ctrl+Z, Ctrl+Shift+Z, Ctrl+Y keybindings
src/ui/components.rs	Updated help overlay with undo/redo keybindings

Test plan

• All 137 tests pass (13 new undo/redo tests)
• Clippy clean
• cargo fmt clean
• Verify Ctrl+Z undoes typing
• Verify Ctrl+Shift+Z / Ctrl+Y redoes
• Verify consecutive typing is grouped into one undo
• Verify newlines create separate undo points
• Verify paste is a single undo unit
• Verify cursor is restored after undo/redo

Closes #15

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