KAN-208: Fix Shift+Y branch copy crash on Linux/Wayland

[fulsomenko]

Summary

• Enable Wayland clipboard support in arboard with wayland-data-control feature
• Add SetExtLinux::wait_until() to give clipboard managers time to take ownership
• Add reusable Banner component for success/error notifications
• Show toast feedback when copying branch name or git checkout command
• Add Nix build dependencies for Wayland/X11 clipboard support

Changes

• crates/kanban-tui/Cargo.toml: Enable wayland-data-control feature
• crates/kanban-tui/src/clipboard.rs: Use wait_until() for clipboard manager handoff
• crates/kanban-tui/src/components/banner.rs: New reusable Banner component
• crates/kanban-tui/src/app.rs: Replace last_error with unified banner system
• crates/kanban-tui/src/ui.rs: Use Banner component for rendering
• default.nix / shell.nix: Add wayland, libxcb, wl-clipboard, xclip deps
• README.md: Document clipboard manager requirement

Note

On Linux, clipboard content is owned by the running process. A clipboard manager (wl-clip-persist, cliphist, clipman, etc.) is required for content to persist after the app exits.