Plugins: manage folders and isolate single-plugin imports

[emaspa]

Changes

• Add Manage folders under Plugins, with add, remove and rescan actions for Windows VST3 and CLAP sources.
• Remove only wrappers associated with the selected folder. Keep original files, unrelated wrappers and wrappers covered by retained folders. Refuse removal while affected inserts remain in the current chains.
• Refresh the catalogue and open pickers after folder removal. Report partial failures and warn when system yabridge's shared registry is in use.
• Copy a selected Windows file or bundle into its own managed folder instead of registering all its siblings. Link plugins already installed inside a Wine prefix so their location and prefix stay intact. Explicit folder picks still register in place.
• Serialize plugin changes so concurrent UI requests cannot share the wrong reply. Add the folder commands and import directory to the documented API.

Verification

• Native-enabled Release build with warnings treated as errors: 0 warnings, 0 errors.
• .NET suite: 453 passed, 3 opt-in desktop tests skipped. The layout test ran separately under Xvfb and passed, including the manager at 480 and 720 pixels.
• Native audio and CLAP tests passed all 16 cases; editor tests passed all 10 cases.
• Real yabridge checks in isolated directories verified registration, in-use refusal, scoped wrapper cleanup and unchanged source hashes.
• A real Kotelnikov import from a folder containing four plugins generated only one wrapper. It still scanned after deleting the test download. Importing the Wine-installed copy preserved its prefix through a link and scanned successfully too.
• Local deployment tested and confirmed working. Mixer and window settings were preserved.

No version bump or release changes.