feat(web): add plugin install dropdown to sidebar with context-based action dispatch

Add '+' dropdown menu to plugins sidebar category with three install
options: marketplace, upload local, and install from GitHub. Use shared
React context (pendingPluginInstallAction) instead of URL params to
reliably trigger install actions across components. Add e.stopPropagation
on all DropdownMenuItem handlers to prevent React portal event bubbling
from triggering parent SidebarMenuButton navigation.

web/src/i18n/locales/ja-JP.ts

@@ -476,6 +476,7 @@
     assetSize: 'サイズ: {{size}}',
     confirmInstall: 'インストールを確認',
     installFromGithubDesc: 'GitHubリリースからプラグインをインストール',
+    goToMarketplace: 'マーケットプレイスへ',
   },
   market: {
     searchPlaceholder: 'プラグインを検索...',