add model loading status, effects preset dropdown, clean up UI

Backend:
- Generation service reports 'loading_model' status only when model
  is not yet in memory, then 'generating' once inference starts
- Migrate hf_offline_patch.py from print() to logging module
- Update ADDING_TTS_ENGINES.md for post-refactor file paths

Frontend:
- HistoryTable shows 'Loading model...' vs 'Generating...' based on step
- FloatingGenerateBox: replace instruct toggle + inline effects editor
  with an effects preset dropdown (third dropdown after language and engine)
- Instruct UI removed for now (form field preserved for future models)
- Remove focus ring from Select component globally

app/src/components/ui/select.tsx

@@ -16,7 +16,7 @@ const SelectTrigger = React.forwardRef<
   <SelectPrimitive.Trigger
     ref={ref}
     className={cn(
-      'flex h-10 w-full items-center justify-between rounded-md border border-input bg-background px-3 py-2 text-sm ring-offset-background placeholder:text-muted-foreground focus:outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50 [&>span]:line-clamp-1',
+      'flex h-10 w-full items-center justify-between rounded-md border border-input bg-background px-3 py-2 text-sm ring-offset-background placeholder:text-muted-foreground focus:outline-none disabled:cursor-not-allowed disabled:opacity-50 [&>span]:line-clamp-1',
       className,
     )}
     {...props}