feat(spx-gui): support deletion of in-progress gen assets and unify collapse/close interaction

[xgopilot]

Background

This issue is recorded from the discussion in PR #2963 (feat(spx-gui): separate generation entry).

After the gen entry is separated, there are two remaining UX issues to address:

Problem 1: No deletion support for in-progress gen assets

When a user starts generating a sprite/backdrop (via AI generation), the in-progress gen item appears in the sprite list / backdrop list as a collapsed thumbnail. Currently, there is no way to delete/cancel a collapsed gen item from the list view.

The GenState (src/components/editor/gen.ts) already has removeSprite(id) and removeBackdrop(id) methods, but they are not exposed in the UI. The SpriteGenItem and BackdropGenItem collapsed thumbnails show loading animations but have no delete/cancel action.

Expected behavior: Users should be able to right-click (or via a context menu) on a collapsed gen item in the sprite/backdrop list to cancel/delete it.

Problem 2: Inconsistent "collapse" vs "close" interaction

Currently, the SpriteGenModal has two behaviors:

• Collapse (@collapse event on SpriteGen.vue): Minimizes the modal and creates a collapsed gen item in the sprite list, keeping the generation running in the background. This is handled via genCollapseHandler.
• Close (clicking the X or mask): Shows a confirmation dialog asking whether to exit (cancel) the generation.

However, BackdropGenModal (BackdropGenModal.vue) does not have a collapse flow — it always shows the exit confirmation dialog and cancels the generation. It is missing the genCollapseHandler prop and the collapse button.

Expected behavior:

• Unify the collapse/close interaction between SpriteGenModal and BackdropGenModal
• Backdrop generation should also support collapsing into a background gen item (similar to sprite gen)
• The "collapse" and "close" semantics should be clearly distinguished in both modals

Files involved

• spx-gui/src/components/asset/gen/sprite/SpriteGenModal.vue — has collapse + close logic
• spx-gui/src/components/asset/gen/backdrop/BackdropGenModal.vue — only has close logic (no collapse)
• spx-gui/src/components/asset/gen/sprite/SpriteGenItem.vue — collapsed sprite gen item (no delete action)
• spx-gui/src/components/asset/gen/backdrop/BackdropGenItem.vue — collapsed backdrop gen item (no delete action)
• spx-gui/src/components/editor/gen.ts — GenState with removeSprite/removeBackdrop methods (unused in UI)
• spx-gui/src/components/asset/index.ts — useBackdropGenModal (missing genCollapseHandler)
• spx-gui/src/components/editor/sprite/SpriteList.vue — renders SpriteGenItem items (no delete handler)
• spx-gui/src/components/editor/stage/backdrop/BackdropsEditor.vue — renders backdrops (no gen item UI)

Acceptance Criteria

• In-progress sprite gen items in the sprite list can be deleted/cancelled from the list view
• In-progress backdrop gen items in the backdrop list can be deleted/cancelled from the list view
• Backdrop generation modal supports "collapse" action (keep running in background) in addition to "close" (cancel)
• Collapse and close interaction is consistent between sprite and backdrop gen modals