Add Library thumbnail previews and a resizable panel

[OmarB97]

Problem

The Library popover lists every Workflows/Exports asset as a bare filename. Once you have more than a handful of exports, the list turns into unreadable filename soup, and the popover has a fixed width, so there's no way to see more of a name or path without scrolling.

Changes

• Each .glb/.gltf row now shows its pre-rendered <name>.thumb.png as an 80px square tile beside the name, loaded through a new workspace:library:thumbnail IPC handler that reuses the same workspace-path validation as the existing workspace:library:read/open handlers (electron/main/artifact-registry-service.ts, electron/preload/electron-api.ts, src/areas/generate/assetLibraryService.ts).
• The Library panel can be resized by dragging its right edge, using the same drag-to-resize pattern already used for the Workflows side panel in this file. The width is clamped to 260-560px and persisted to localStorage so it survives restarts (src/areas/generate/assetLibraryUi.ts, src/areas/generate/GeneratePage.tsx).

Degrading without a thumbnail

If <name>.thumb.png doesn't exist, or fails to read, the IPC handler returns { success: false } with no error payload — a missing thumbnail is a normal, silent condition, not a surfaced error. The row then renders exactly as it did before this change: no image element, no broken-image icon, just the existing text-only row.

Testing

• npm run lint — clean
• npm test (test:py + test:node, including all existing assetLibrary*.test.ts files) — all pass, plus new tests covering the thumbnail read path (missing file, non-mesh asset, unsafe/traversal path) and the panel-width clamp/persistence helpers
• npm run build — renderer, main, and preload all build successfully