feat(linux): precise terminal focus via WINDOWID + custom terminal support

[tearfu1]

Summary

• Precise window focus on Linux: Read WINDOWID from /proc/<pid>/environ instead of matching by terminal server PID. This fixes the issue where Focus Terminal would activate the wrong window/tab when multiple terminal windows are open.
• Kitty remote control: Tab-level focus via kitty @ focus-window --match pid:<pid> for kitty users.
• Custom terminal binary: Replace hardcoded terminal dropdown in Settings with a text input + datalist suggestions. Users can specify any terminal binary name or full path (e.g. ghostty, /usr/bin/alacritty).
• System default detection: Auto-detect x-terminal-emulator symlink target on Debian/Ubuntu.
• Smart launch flags: Detect terminal type by basename — gnome-terminal gets --, wezterm gets start --, everything else gets generic -e.

What was broken

1. focusTerminalByPid() on Linux walked the parent chain to find the terminal server PID, then matched it against wmctrl -l -p. Since terminal servers (e.g. gnome-terminal-server) own ALL windows with a single PID, it always focused the first window — not the one with the active session.
2. The terminal selector was a fixed dropdown with 5 terminals. Users of ghostty, wezterm, terminator, tilix, foot, etc. had no way to configure their terminal.
3. The default launch terminal was hardcoded to gnome-terminal.

How it works now

Focus (WINDOWID approach)

agent PID → read /proc/<pid>/environ → find WINDOWID=12345
  → xdotool windowactivate 12345 (or wmctrl -ia 0x00003039)
  → falls back to PID chain matching if WINDOWID not found

Custom terminal

Settings now shows a text input with autocomplete suggestions from detected binaries. The value is sent as-is to the server, which determines launch flags by path.basename().

Test plan

• Linux: Focus Terminal with multiple terminal windows open → correct window activates
• Linux: Focus Terminal with tabs → correct window activates (tab-level for kitty with remote control enabled)
• Settings: type custom terminal path → Resume opens in that terminal
• Settings: datalist shows detected terminals as suggestions
• macOS: no regressions (darwin code path unchanged)