feat(webview): structured assistant content model (ChatContentPart + contentParts)

Extracted from #39. Foundation for thinking and tool-call rendering: assistant messages become an ordered list of typed parts instead of one growing markdown string.

Scope: `ChatContentPart` union (TextPart | ThinkingPart | ToolCallPart) and optional `ChatMessage.contentParts` (purely additive); `useChat` reducer assembles parts (coalesce same-type chunks into the open streaming part, finalize on type switch, merge tool-call updates by id, keep a flattened text mirror for copy/history); `AssistantMessage` renders the parts loop with the legacy plain-content path as fallback.

Must reconcile with the error-path changes #49 made in `useChat`.

**Size**: M | **Depends on**: nothing | **Tier**: 1 (foundation; blocks the thinking and tool-call issues)