fix: hide AssistantFAB during spec creation mode

The Chat AI Assistant button (AssistantFAB) was appearing on top of the
full-screen spec creation chat overlay, causing a visual bug where the
button would overlap with the Send input area.

Changes:
- Add onStepChange callback prop to NewProjectModal to notify parent
  when the modal step changes
- Add onSpecCreatingChange callback prop to ProjectSelector to propagate
  spec creation state up to App.tsx
- Add isSpecCreating state to App.tsx to track when spec creation chat
  is active
- Update AssistantFAB render condition to include !isSpecCreating
- Disable 'A' keyboard shortcut during spec creation mode

The fix propagates the spec creation state through the component
hierarchy: NewProjectModal -> ProjectSelector -> App.tsx, allowing
the FAB to be hidden when step === 'chat' in the new project modal.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/components/ProjectSelector.tsx

@@ -10,6 +10,7 @@ interface ProjectSelectorProps {
   selectedProject: string | null
   onSelectProject: (name: string | null) => void
   isLoading: boolean
+  onSpecCreatingChange?: (isCreating: boolean) => void
 }
 
 export function ProjectSelector({
@@ -17,6 +18,7 @@ export function ProjectSelector({
   selectedProject,
   onSelectProject,
   isLoading,
+  onSpecCreatingChange,
 }: ProjectSelectorProps) {
   const [isOpen, setIsOpen] = useState(false)
   const [showNewProjectModal, setShowNewProjectModal] = useState(false)
@@ -166,6 +168,7 @@ export function ProjectSelector({
         isOpen={showNewProjectModal}
         onClose={() => setShowNewProjectModal(false)}
         onProjectCreated={handleProjectCreated}
+        onStepChange={(step) => onSpecCreatingChange?.(step === 'chat')}
       />
 
       {/* Delete Confirmation Dialog */}