feat(webview): render streaming cursor inline at end of streamed text

Extracted from #39. The blinking cursor currently renders after the whole markdown block, landing on its own line.

Relocate the caret into the last text-bearing element (p, li, blockquote, pre code, headings) via an idempotent DOM effect in `MarkdownRenderer`, plus the `.markdown-content-bubble { white-space: pre-wrap }` fix so user bubbles preserve line breaks. Known trade-off to keep in mind: this is an imperative DOM mutation into React-managed output (react-markdown has no cursor seam); tighten the effect deps when porting.

**Size**: S | **Depends on**: nothing | **Tier**: 0 (quick win)