feat: detect MCP server status and show warning in status bar

Add check_mcp_status Tauri command that detects whether the MCP server
is installed, Claude CLI is missing, or config needs setup. The status
bar shows a warning badge (MCP ⚠) when not installed, clickable to
trigger install. Also available via command palette "Install MCP Server".

Replaces useMcpRegistration with useMcpStatus which combines detection
and registration in a single hook.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useAppCommands.ts

@@ -64,6 +64,8 @@ interface AppCommandsConfig {
   onRestoreGettingStarted?: () => void
   isGettingStartedHidden?: boolean
   vaultCount?: number
+  mcpStatus?: string
+  onInstallMcp?: () => void
 }
 
 /** Sets up keyboard shortcuts, command registry, menu events, and keyboard navigation. */
@@ -172,6 +174,8 @@ export function useAppCommands(config: AppCommandsConfig): CommandAction[] {
     onRestoreGettingStarted: config.onRestoreGettingStarted,
     isGettingStartedHidden: config.isGettingStartedHidden,
     vaultCount: config.vaultCount,
+    mcpStatus: config.mcpStatus,
+    onInstallMcp: config.onInstallMcp,
   })
 
   useKeyboardNavigation({