feat(webview): remove history-message truncation

[cloud-on-prem]

Extracted from #39. Gated on decision #63.

Delete the MAX_LINES_COLLAPSED / truncateContent / "Show more (N lines)" machinery in AssistantMessage and UserMessage so replayed history renders full-length. Net simplification of both components.

Size: S | Depends on: decision #63 | Tier: 3