feat(ui): add searchable base model dropdown with filename inference in model modal

Replace native <select> with a searchable dropdown that:
- Filters options as the user types
- Shows filename-inferred suggestions at the top in a "Suggested" section
- Supports keyboard navigation (ArrowUp/Down/Enter/Escape)
- Allows typing custom values not in the list
- Removes dead .base-model-selector CSS

Adds 3 new i18n keys (baseModelSearchPlaceholder, baseModelSuggested,
baseModelNoMatch) with translations for all 9 locales.

locales/en.json

@@ -1347,7 +1347,10 @@
                 "additionalNotes": "Additional Notes",
                 "notesHint": "Press Enter to save, Shift+Enter for new line",
                 "addNotesPlaceholder": "Add your notes here...",
-                "aboutThisVersion": "About this version"
+                "aboutThisVersion": "About this version",
+                "baseModelSearchPlaceholder": "Search base model…",
+                "baseModelSuggested": "Suggested",
+                "baseModelNoMatch": "No matching base models"
             },
             "notes": {
                 "saved": "Notes saved successfully",