feat(plugins): install a plugin from a chosen directory

[sunrisegm]

Summary

• The plugins page opens a native folder dialog (rfd: NSOpenPanel on macOS, IFileDialog on Windows, portal/GTK on Linux) in the same process that copies the plugin.
• The server copies a snapshot into plugins/installed/<plugin id>, validates it the same way as the built-in plugins, and does not follow symlinks outside the chosen directory.
• A built-in plugin id cannot be replaced. Installing the same user plugin again asks for confirmation and keeps the saved accounts.

Test plan

• Initialize the plugin runtime, then use Add plugin and choose a plugin directory
• Cancel the dialog and confirm nothing is installed
• Add the same plugin again and confirm replacement keeps saved accounts
• Confirm a built-in plugin id is rejected
• Repeat the dialog on macOS, Windows, and Linux

Made with Cursor

[sunrisegm]

Branch renamed to ADD-CUSTOM-PLUGINS/feat/install-plugin. Continued in #495.