feat: rendered markdown, Mermaid diagrams, Google Docs-style comments, dark mode

[Ronkahn21]

Problem

diffx lacked several key features for a complete local code review experience:

• .md files could only be reviewed as raw diffs with no rendered preview
• Mermaid diagrams in markdown were invisible
• Comments on markdown files couldn't anchor to specific text passages in the rendered view
• Multi-line comment selection gave no visual feedback about where the range started
• No dark mode

Solution

• Rendered markdown view: toggle any .md file between Diff and Rendered with a toolbar button
• Mermaid diagrams: fenced mermaid blocks render as live diagrams in the rendered view (lazy-loaded)
• Google Docs-style comments: select any text passage in the rendered view → floating + → comment anchored to the highlighted text with right-margin bubbles
• Multi-line diff comments: press-drag the gutter + across lines to select a range; a ghost + stays pinned to the start line while dragging
• Dark mode: sun/moon toggle in toolbar, persisted to localStorage
• /diffx-finish-review skill updated: documents both comment types with all anchor fields