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/en-US.ts

@@ -476,6 +476,7 @@ const enUS = {
     assetSize: 'Size: {{size}}',
     confirmInstall: 'Confirm Install',
     installFromGithubDesc: 'Install plugin from GitHub Release',
+    goToMarketplace: 'Go to Marketplace',
   },
   market: {
     searchPlaceholder: 'Search plugins...',

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...',

web/src/i18n/locales/zh-Hans.ts

@@ -452,6 +452,7 @@ const zhHans = {
     assetSize: '大小: {{size}}',
     confirmInstall: '确认安装',
     installFromGithubDesc: '从 GitHub Release 安装插件',
+    goToMarketplace: '前往插件市场',
   },
   market: {
     searchPlaceholder: '搜索插件...',

web/src/i18n/locales/zh-Hant.ts

@@ -445,6 +445,7 @@ const zhHant = {
     assetSize: '大小: {{size}}',
     confirmInstall: '確認安裝',
     installFromGithubDesc: '從 GitHub Release 安裝插件',
+    goToMarketplace: '前往外掛市場',
   },
   market: {
     searchPlaceholder: '搜尋插件...',