feat(models): add shared searchable base model picker and overhaul bulk base model modal

- Extract a shared BaseModelPicker (search, keyboard navigation,
  filename-based suggestions, dynamic API models such as MiniMax H3
  under 'Other (API)') used by both the single-model metadata modal
  and the bulk base model modal
- Rework the bulk base model modal into a dedicated inline-list
  layout: fixed modal size, sticky-free footer with app-standard
  modal-actions/primary-btn/cancel-btn buttons, and an inline option
  list that scrolls itself instead of an overlay dropdown covering
  the footer
- Selecting an option in change mode now filters the list to the
  selection instead of resetting and scroll-jumping to it
- Restore opaque sticky section headers in the bulk modal so scrolled
  items no longer bleed through

static/js/utils/constants.js

@@ -87,6 +87,10 @@ export const BASE_MODELS = {
     UNKNOWN: "Other"
 };
 
+// Window event dispatched after dynamic base models are (re)loaded from the API.
+// Pickers listen for it to refresh their option lists when data arrives late.
+export const BASE_MODELS_UPDATED_EVENT = 'lora-manager:base-models-updated';
+
 // Custom dataTransfer MIME type tagging internal model-card drags (move-to-folder).
 // Preview-drop handlers use it to ignore drags that did not come from the OS file system.
 export const MODEL_CARD_DRAG_MIME_TYPE = 'application/x-lora-manager-model-card';