feat(desktop): start Windows/Linux smart paste

[senseb]

Adds source-level Windows/Linux desktop smart-paste support.

• Enables Advanced mode outside macOS.
• Registers Ctrl+V through the Tauri global shortcut plugin on Windows/Linux.
• Temporarily unregisters Ctrl+V before emitting paste to avoid recursion.
• Uses VS Code extension terminal state for Windows/Linux target-aware detection.
• Adds Linux pass-through paste via xdotool/wtype/ydotool.
• Updates README/desktop docs to describe the dev-build scope.

Validation already run locally:

• cargo test --all-targets
• cargo clippy --all-targets --all-features -- -D warnings
• cargo check --manifest-path desktop/src-tauri/Cargo.toml
• cargo clippy --manifest-path desktop/src-tauri/Cargo.toml --all-targets --all-features -- -D warnings
• node --check desktop/ui/main.js
• git diff --check

Next validation:

• Run desktop npm ci && npm run build:no-bundle on Windows and Linux runners.
• Manual tray/Ctrl+V/VS Code target validation on interactive Windows and Linux desktops.