Add modern timeline UI with thumbnail previews

[desperadoxhy]

Implements a modern, Apple News-style horizontal split layout for the timeline, addressing #4981 (Modern UI Refresh for Lists).

What's added

A new timeline cell: the left side shows the feed icon, title, and summary; the right side shows a thumbnail when the article has an image.

macOS — TimelineModernCellView

• Manual frame layout via TimelineModernCellLayout (per the project guideline that table-view cells don't use Auto Layout, for performance)
• isFlipped so coordinates run top-down
• Reuse-safe thumbnail loading (NSTableView has no prepareForReuse callback)

iOS — MainTimelineModernCell

• Auto Layout self-sizing cell

Shared — ArticleImageExtractor

• Resolves an article's image URL: the declared imageURL, or the first `` / image URL found in the content
• Thumbnails are center-cropped to a square and cached

Behavior (both platforms)

• Title wraps to at most 2 lines; the summary line count adjusts so a 2-line title gets a 1-line summary and a 1-line title gets a 2-line summary
• Read state dims the title color
• Metadata shows the feed name

Coverage of #4981

• ✅ Thumbnail previews in the list
• ⚠️ Feed icon is 20pt here (the issue suggests 16pt) — happy to adjust
• ❌ Unread marker refinement is not included in this PR

Notes

• Builds clean on macOS and iOS (no warnings from these files)
• All new classes are `final`; no `@objc`, no KVO, no console output
• Local-only changes (signing config, an unrelated xib) were intentionally excluded

Refs #4981.