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).