[i18n] 5 areas bypass translation system -- context menus, toasts, settings labels, search headers, widget placeholders

## Description

There are 5 areas in the ComfyUI frontend where UI text is hardcoded in English, bypassing the translation system entirely. The toolbar/queue/status bar strings in `ui.ts` are already tracked in #12263.

## Areas not covered by #12263

### 1. Context menus (3 files)
- `LGraphCanvas.ts` -- node context menu items (~30+ strings)
- `groupOptions.ts` -- group operation menu items
- `litegraphService.ts` -- graph-level context menu items
- Total: ~50+ hardcoded strings

### 2. Toast notifications (api.ts, app.ts)
- Success/error/info toasts for subscription workflows, queue operations, and API responses
- Messages like "Queue Prompt sent", "Interrupted", etc.

### 3. Settings category labels (useSettingUI.ts)
- Category names and section headers in the settings modal
- Labels like "Appearance", "Performance", "Advanced"

### 4. Search/registry column headers (algoliaSearchProvider.ts, registrySearchProvider.ts)
- Table column headers in the node search and registry browser
- Strings like "Name", "Author", "Description", "License"

### 5. Widget placeholders (MultiSelectWidget.vue)
- Placeholder text and empty-state messages in multi-select widget components

## Expected behavior

All UI strings visible to users should be routed through the i18n translation system so they can be localized.