Add opt-in auto paste setting

[Sarthak-Raghav]

Recopied's current flow is click-to-copy: selecting an item places it on the clipboard and hides the popup, and the user presses Ctrl+V themselves. This PR adds an optional Windows-style flow on top: an Auto Paste setting that simulates Ctrl+V in the previously focused window once the popup hides. Off by default.

• New auto_paste setting (serde default = false, so existing settings.json files are unaffected)
• paste_item spawns a background thread after hiding: 150ms focus-settle delay, then xdotool key --clearmodifiers ctrl+v — consistent with the existing xclip usage; X11 only
• New set_auto_paste command + a settings UI toggle styled to match the notification toggles, with a caption documenting the xdotool requirement and the terminal (Ctrl+Shift+V) caveat

Motivation: replicating the Windows Win+V flow. I originally built this externally with xdotool/clipnotify and hit focus races that only an in-app implementation avoids. Addresses #1 — that report shows users expect click-to-paste and read its absence as a bug.
Tested on Linux Mint 22 Cinnamon (X11): top item, lower items, and Esc-dismiss all behave correctly; auto-paste lands in the previously focused window. (Built with AI assistance; my Rust knowledge is pretty limited and I am also new to Linux)