Add drag-and-drop plugin validation to the main window

[jcredland]

Summary

• Drop a .vst3 / .vst / .component / .dll / .so / .clap anywhere on the pluginval window to validate it.
• Shows a translucent rounded-rectangle overlay with a "Drop plug-in to validate" label while dragging an accepted file.
• Routes through Validator::validate(StringArray, options) — the same flow used by Test File…, so it picks up the current strictness level, timeout, sample rates, etc. Console tab is auto-activated when validation starts (existing behaviour).
• Mixed drops accept the plugin files and ignore the rest; drops with no plugin files are a no-op.

Test plan

• Drag a .vst3 onto the window — highlight appears, drop starts validation.
• Drag a non-plugin file (e.g. .txt) — no highlight, drop is ignored.
• Drag multiple plugins at once — all are queued via a single validate call.
• Drag onto the Console tab as well as the Plugin List tab — both trigger validation (plugin list table doesn't intercept drops).

[sudara]

Thanks for this!

I QA'd this and looked through it.

The prior behavior is that a drag would add a plugin to the list (vs. run validation). It was the main way I added plugins (dropping them in from finder). I do think that still makes sense to add, even though it's also useful that it starts a run. We could do both with a change like this: 6ec3eb6

The other thing is that with the run-on-drop behavior, I can reliably beachball pluginval by dropping a plugin in there while another is scanning. Something like this would fix: c7b155a

Otherwise looks and works great, happy to merge with my commits as followups or wait for changes.