Introduce block-level content model for rich message rendering

[subpop]

The HTML parser (MatrixHTMLParser) currently produces a single NSAttributedString for the entire message body. This works well for inline formatting but limits how we can render block-level elements that don't map cleanly to attributed string attributes -- tables being the most immediate example (currently rendered as tab-delimited text per #93), but also relevant for future support of <img> with mxc:// sources, <details>/<summary>, and potentially math blocks.

Proposed approach:

Introduce an intermediate block model that splits parsed output into typed segments:

enum MessageBlock: Identifiable {
    case richText(NSAttributedString)
    case table(MessageTable)
    // Future: .image, .details, .math, etc.
}

struct MessageTable {
    let caption: NSAttributedString?
    let headers: [NSAttributedString]
    let alignments: [HorizontalAlignment]
    let rows: [[NSAttributedString]]
}

The parser would return [MessageBlock] instead of NSAttributedString?. When it encounters a <table>, it flushes accumulated text as a .richText block, builds a structured MessageTable from the table's cells (preserving inline formatting within each cell as NSAttributedString), then continues accumulating text into a new .richText block.

On the view side, MessageBubbleContent would iterate over the blocks within a single bubble, rendering .richText blocks with the existing MessageTextView and .table blocks with a new SwiftUI Grid-based MessageTableView. This pattern is proven in Quack, which uses the same block-level split to render Markdown tables as SwiftUI grids alongside AttributedString text.

Key changes:

• MatrixHTMLParser.parse() returns [MessageBlock] instead of NSAttributedString?
• Parser flushes accumulated NSMutableAttributedString when entering/leaving table context, managing blockquote depth, list state, and style stacks across the split
• New MessageTable model type and MessageTableView (SwiftUI Grid)
• MessageBubbleContent.textContent renders a ForEach over blocks instead of a single MessageTextView
• Parse caches (htmlCache, markdownCache) change value type from NSAttributedString to [MessageBlock]
• Table cells containing inline formatting (bold, links, code) are preserved as NSAttributedString within the MessageTable model

Benefits over the current tab-delimited approach:

• Proper column alignment independent of content width
• Bold headers, column alignment markers (:---:)
• Alternating row backgrounds for readability
• Text selection within individual cells
• Extensible to other block-level elements without further architectural changes

Reference: Quack's implementation lives in MarkdownRenderer.renderBlocks(), MarkdownContentView, and MarkdownTableView.