feat: add resizable panes with draggable dividers

[randlee]

Summary

• Add draggable vertical dividers between panes for user-controlled sizing
• Implement terminal hardening fixes from ARCH-CODEX architecture review
• Remove dead code from previous terminal element refactor

Changes

Resizable Panes

• ResizeDragState struct tracks drag operations (divider index, start position, ratios)
• render_divider() creates 6px hitbox with col-resize cursor and visual feedback
• flex_basis(relative(ratio)) layout replaces equal flex_1() sizing
• Double-click divider resets to default ratios (1:2:1)
• MIN_PANE_WIDTH (150px) enforced during resize
• Ratios persist via existing debounced save mechanism

Terminal Hardening (ARCH-CODEX)

• HIGH: Add minimum-width guard (2 columns) to prevent alacritty rendering crashes
• MEDIUM: Fix Alt+key handling to prepend ESC for shell shortcuts (Alt+b, Alt+f, etc.)
• LOW: Remove unused update_terminal_snapshot, build_lines_from_content, and rendered_lines

Files Changed

• src/ui/workspace.rs - Main resizable panes implementation
• src/terminal/element.rs - Minimum-width guard
• src/terminal/pane.rs - Alt+key fix, dead code removal
• src/terminal/tab.rs - Remove unused field
• docs/design/resizable-panes.md - Design document

Test plan

• Build passes: cargo check
• Clippy clean: cargo clippy --all-targets
• Tests pass: cargo test (68 tests)
• Manual: Drag dividers to resize panes
• Manual: Double-click divider to reset ratios
• Manual: Test Alt+b/Alt+f in terminal for word navigation
• Manual: Resize window very narrow to test minimum-width guard

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