refactor: Change manager flag from --disable-manager to --enable-manager (#5635)

## Summary
- Updated frontend to align with backend changes in ComfyUI core PR
#7555
- Changed manager startup argument from `--disable-manager` (opt-out) to
`--enable-manager` (opt-in)
- Manager is now disabled by default unless explicitly enabled

## Changes
- Modified `useManagerState.ts` to check for `--enable-manager` flag
presence
- Inverted logic: manager is disabled when the flag is NOT present
- Updated all related tests to reflect the new opt-in behavior
- Fixed edge case where `systemStats` is null

## Related
- Backend PR: https://github.com/comfyanonymous/ComfyUI/pull/7555

## Test Plan
- [x] All unit tests pass
- [x] Verified manager state logic with different flag combinations
- [x] TypeScript type checking passes
- [x] Linting passes

🤖 Generated with [Claude Code](https://claude.ai/code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5635-refactor-Change-manager-flag-from-disable-manager-to-enable-manager-2726d73d36508153a88bd9f152132b2a)
by [Unito](https://www.unito.io)

src/composables/useManagerState.ts

@@ -42,7 +42,12 @@ export function useManagerState() {
       )
 
       // Check command line args first (highest priority)
-      if (systemStats.value?.system?.argv?.includes('--disable-manager')) {
+      // --enable-manager flag enables the manager (opposite of old --disable-manager)
+      const hasEnableManager =
+        systemStats.value?.system?.argv?.includes('--enable-manager')
+
+      // If --enable-manager is NOT present, manager is disabled
+      if (!hasEnableManager) {
         return ManagerUIState.DISABLED
       }