fix(slackdesktop): detect flatpak and snap Slack config dirs on Linux

PR #106's desktop-app auth only looked at $XDG_CONFIG_DIR/Slack or
~/.config/Slack, so users of the Slack flatpak (and snap) got
'slack desktop app config directory not found' even when signed in.

Check candidate locations in order on Linux:
  1. $XDG_CONFIG_HOME/Slack
  2. $XDG_CONFIG_DIR/Slack (legacy)
  3. ~/.config/Slack (native)
  4. ~/.var/app/com.slack.Slack/config/Slack (flatpak)
  5. ~/snap/slack/current/.config/Slack (snap)

First existing dir wins; native install is preferred over packaged.
Cookie decryption needs no changes: the flatpak stores its 'Slack Safe
Storage' secret on the host secret service with the same libsecret
schema, and the v10 'peanuts' fallback covers confined setups.

Fixes #123