feat(plugin): uninstall + clean upgrade/reinstall of external plugins

Add an Uninstall button on installed external plugin cards that deletes
the package folder (effective next restart — the library stays resident
for the session). Reinstalling or upgrading via the marketplace now
clears any stale native library with a different name before writing the
new one, so the loader always picks the freshly installed binary.

Part of the #100 extensibility epic (phase 2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -1073,6 +1073,8 @@ pub enum Message {
     PluginInstall(String),
     /// Result of an install: the plugin id, or an error message.
     PluginInstalled(Result<String, String>),
+    /// Delete an installed plugin's folder (effective next restart).
+    PluginUninstall(String),
     // ── Point Style (DDPTYPE) dialog ──────────────────────────────────────
     /// Set the full PDMODE value from a glyph-grid cell.
     PointStyleSetMode(i16),