Remove deprecated error getters from ComfyApp (app.ts)

## Context
PR #9060 extracted error-related state from executionStore to executionErrorStore, leaving deprecated getters in ComfyApp for backward compatibility.

## Current Deprecated Getters
- `app.lastNodeErrors` → currently points to `useExecutionErrorStore().lastNodeErrors`
- `app.lastExecutionError` → currently points to `useExecutionErrorStore().lastExecutionError`

## Problem
Custom extensions cannot easily access `useExecutionErrorStore()` at runtime. The current deprecation messages are not actionable for extension authors.

## Proposed Solution
**Phase 1** (should be done in a follow-up PR to #9060):
- Expose error properties through `app.extensionManager` public API (currently at `app.extensionStore`)
- Update deprecation messages to point extension authors to `app.extensionStore.lastNodeErrors` etc.
- Reference: https://github.com/Comfy-Org/ComfyUI_frontend/blob/bb40ffae3c2d8e5fdf0c0fb1255a633283892f6b/src/stores/workspaceStore.ts#L89-L110

**Phase 2** (this issue - target: 6-12 months after Phase 1):
- Remove deprecated getters from `app.ts` entirely
- By then, extensions should have migrated to `app.extensionStore` API
- Audit extension usage before removal

## Timeline
- Target removal: August 2026 - February 2027 (6-12 months from Feb 2026)
- Prerequisite: Complete Phase 1 first

## Related
- PR: #9060
- Discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9060#discussion_r2836771402
- Requested by: @christian-byrne

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-9066-Remove-deprecated-error-getters-from-ComfyApp-app-ts-30e6d73d365081fc9ec9eb53152121eb) by [Unito](https://www.unito.io)