feat: App mode saving rework (#9338)

## Summary

Change app mode changes to be written directly to the workflow on change
instead of requiring explicit save via builder.
Temporary: Adds `.app.json` file extension to app files for
identification since we don't currently have a way to identify them with
metadata
Removes app builder save dialog and replaces it with default mode
selection

## Changes

- **What**: 
- ensure all save locations handle app mode
- remove dirtyLinearData and flushing

- **Breaking**: 
- if people are relying on workflow names and are converting to/from app
mode in the same workflow, they will gain/lose the `.app` part of the
extension

## Screenshots (if applicable)

<img width="689" height="84" alt="image"
src="https://github.com/user-attachments/assets/335596ee-dce9-4e3a-a7b5-f0715c294e41"
/>

<img width="421" height="324" alt="image"
src="https://github.com/user-attachments/assets/ad3cd33c-e9f0-4c30-8874-d4507892fc6b"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9338-feat-App-mode-saving-rework-3176d73d3650813f9ae1f6c5a234da8c)
by [Unito](https://www.unito.io)

src/platform/workflow/management/stores/comfyWorkflow.ts

@@ -24,31 +24,6 @@ export interface PendingWarnings {
   }
 }
 
-type LinearModeTarget = { extra?: Record<string, unknown> | null } | null
-
-export function syncLinearMode(
-  workflow: ComfyWorkflow,
-  targets: LinearModeTarget[],
-  options?: { flushLinearData?: boolean }
-): void {
-  for (const target of targets) {
-    if (!target) continue
-    if (workflow.initialMode === 'app' || workflow.initialMode === 'graph') {
-      const extra = (target.extra ??= {})
-      extra.linearMode = workflow.initialMode === 'app'
-    } else {
-      delete target.extra?.linearMode
-    }
-    if (options?.flushLinearData && workflow.dirtyLinearData) {
-      const extra = (target.extra ??= {})
-      extra.linearData = workflow.dirtyLinearData
-    }
-  }
-  if (options?.flushLinearData && workflow.dirtyLinearData) {
-    workflow.dirtyLinearData = null
-  }
-}
-
 export class ComfyWorkflow extends UserFile {
   static readonly basePath: string = 'workflows/'
   readonly tintCanvasBg?: string
@@ -77,12 +52,6 @@ export class ComfyWorkflow extends UserFile {
    * Takes precedence over initialMode when present.
    */
   activeMode: AppMode | null = null
-  /**
-   * In-progress builder selections not yet persisted via save.
-   * Preserved across tab switches, discarded on exitBuilder.
-   */
-  dirtyLinearData: LinearData | null = null
-
   /**
    * @param options The path, modified, and size of the workflow.
    * Note: path is the full path, including the 'workflows/' prefix.