feat: custom collection classes and folder hub

[3-pr]

Description

Adds a dedicated "Folder Hub" view for collection folders that contain multiple sources. Previously, opening a folder with more than one source silently picked the first source and discarded the rest. Now, multi-source folders open into a proper hub view.

Key Changes

• New Folder Hub view (src/views/folder-hub.tsx): renders multiple sources within a folder as either stacked horizontal rows ("Classic") or a tabbed infinite-scroll grid ("Grid"), depending on user preference.
• Layout toggle: users can set a per-folder layout preference from the "Edit Folder Images" modal, or switch on the fly via quick-toggle icons in the Folder Hub header — both write to the same persisted state.
• Lazy loading: Classic rows fetch their data only when scrolled into view (IntersectionObserver), and Grid mode validates sources sequentially in the background to hide empty/broken tabs without spamming concurrent requests.
• Background visuals: folders with a cover image or animated preview now render it as a blurred, faded hero background behind the hub content.
• Parsing improvements (src/lib/custom-sources.ts): more robust inference of provider/source type from imported collection data, with malformed entries (missing required fields) safely skipped instead of guessed.

Files Changed

• src/lib/custom-sources.ts
• src/components/edit-folder-images-modal.tsx
• src/views/home.tsx
• src/views/folder-hub.tsx (new)

Verification

• pnpm run typecheck passes with 0 errors.
• Manually verified: multi-source folders open the Folder Hub; layout toggle persists per folder from both the modal and the header; single-source folders continue to behave as before (regression-checked); tab switching in Grid mode loads correct, non-duplicated items with working infinite scroll.