feat: update user-facing 'Simple Mode' terminology to 'App Mode' (#8590)

## Summary
Updates all user-facing instances of 'Simple Mode' / 'Linear Mode' to
'App Mode' as discussed in PR #8562.

## Changes
- Updated command label in `useCoreCommands.ts`
- Updated `src/locales/en/commands.json`
- Updated all 12 locale `main.json` files with:
  - `linearMode.linearMode` → "App Mode" (translated)
  - `linearMode.beta` → "App Mode in Beta - Feedback" (translated)
  - `Toggle Simple Mode` command → `Toggle App Mode` (translated)

## Languages Updated
en, zh, zh-TW, ja, ko, fr, es, pt-BR, ru, tr, ar, fa

## Notes
Internal variable names (`linearMode`, `canvasStore.linearMode`) remain
unchanged to avoid breaking changes.

Fixes #8577

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Style**
* Renamed application terminology from "Simple Mode" to "App Mode" in
user interface labels, mode descriptions, and mode toggle command.
Updates applied consistently across all 12 supported language
translations to ensure consistent terminology throughout the
application.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8590-feat-update-user-facing-Simple-Mode-terminology-to-App-Mode-2fc6d73d36508160a19af70b228b0a7e)
by [Unito](https://www.unito.io)

src/composables/useCoreCommands.ts

@@ -1233,7 +1233,7 @@ export function useCoreCommands(): ComfyCommand[] {
     {
       id: 'Comfy.ToggleLinear',
       icon: 'pi pi-database',
-      label: 'Toggle Simple Mode',
+      label: 'Toggle App Mode',
       function: (metadata?: Record<string, unknown>) => {
         const source =
           typeof metadata?.source === 'string' ? metadata.source : 'keybind'