Centralize upload helper functions into shared utility

## Context

Multiple upload helper implementations are duplicated across the codebase, making it difficult to maintain and refactor upload logic consistently.

## Locations with duplicated upload functionality

- `src/composables/node/useNodeImageUpload.ts:20-49` (most generic implementation)
- `src/composables/maskeditor/useMaskEditorSaver.ts:266-299`
- `src/components/common/BackgroundImageUpload.vue:55-74`
- `src/extensions/core/webcamCapture.ts`
- `src/extensions/core/uploadAudio.ts:28-73`
- `src/services/audioService.ts`
- `src/renderer/extensions/vueNodes/widgets/components/WidgetSelectDropdown.vue`

## Proposal

Centralize these implementations into a single shared utility or service that all modules can import from. This will:
- Reduce code duplication
- Make future changes to upload logic easier to maintain
- Ensure consistent behavior across all upload operations
- Simplify testing and error handling

## References

- Identified in PR #8129: https://github.com/Comfy-Org/ComfyUI_frontend/pull/8129
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/8129#discussion_r2701437891
- Requested by: @christian-byrne

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