Manager: replace the ugly default node-pack banner/thumbnail for packs without an image

## Problem

In the Manager, node packs that have no `banner_url` and no `icon` fall back to a default banner image that looks bad. Because many packs lack artwork, this default shows up frequently across the pack grid and the pack detail panel, making the Manager feel low-quality.

## Where it lives

- Fallback is centralized in [PackBanner.vue](https://github.com/Comfy-Org/ComfyUI_frontend/blob/main/src/workbench/extensions/manager/components/manager/packBanner/PackBanner.vue), used by both the pack cards and the detail panel.
- The asset is `/assets/images/fallback-gradient-avatar.svg` (`DEFAULT_BANNER`).
- It is shown when both `banner_url` and `icon` are missing, and also when a provided image fails to load (`@error`).

## Goal

Replace the current default with something that looks good and intentional when repeated many times across a grid. It should:
- Look clean and on-brand at the card aspect ratio (`aspect-7/3`) and in the larger detail banner.
- Avoid looking broken/empty; ideally feel like a deliberate placeholder rather than a missing image.
- Stay visually consistent whether it's the no-image case or the image-load-error case.

## Possible directions (to discuss)
- A refreshed, subtler default graphic (better gradient/pattern), OR
- A generated placeholder derived from the pack (e.g. initials/identicon from the pack name/id) so repeated cards look varied rather than identical, OR
- A neutral branded placeholder with the Comfy mark.

Design input wanted - consult the Comfy Design Standards before implementing.

## Acceptance criteria
- Node packs without artwork render an attractive, intentional placeholder in both the grid and the detail view.
- The image-load-error path uses the same improved placeholder.