feat: show legacy manager search tip in no-results empty state (#8537)

## Summary

Show a non-intrusive tip in the no-results empty state when users search
legacy manager-related terms in the new manager UI.

## Changes

- **What**: Add `useLegacySearchTip` composable that detects when search
query matches legacy manager keywords ("manager", "comfyui-manager",
etc.) and appends a tip to the empty state message suggesting the
`--enable-manager-legacy-ui` flag
- **Integration**: Tip appears as secondary muted text below "Try a
different search" message when no results found
- **Tests**: 9 test cases covering detection logic and edge cases

## Review Focus

- Keyword list covers common legacy manager search terms
- Non-intrusive approach: tip only shows in no-results state, no dismiss
button needed

Fixes COM-12509

src/locales/en/main.json

@@ -290,6 +290,7 @@
     "legacyMenuNotAvailable": "Legacy manager menu is not available, defaulting to the new manager menu.",
     "legacyManagerUI": "Use Legacy UI",
     "legacyManagerUIDescription": "To use the legacy Manager UI, start ComfyUI with --enable-manager-legacy-ui",
+    "legacyManagerSearchTip": "Looking for ComfyUI-Manager? You can enable the legacy manager UI by starting ComfyUI with the --enable-manager-legacy-ui flag.",
     "failed": "Failed",
     "failedToInstall": "Failed to Install",
     "installError": "Install Error",