fix(ui): preserve single line breaks in submitted user messages

[joestump]

A user message typed as

a
b

c

rendered in the chat as "a b" on one line. Only the display was affected — the content persisted to history and sent to the model was always correct.

User input is rendered through the shared Markdown renderer, where a lone newline is a soft break and adjacent lines are joined into one paragraph. That is correct for Markdown source, but user input is authored in a plain textarea: a newline there is a line the user deliberately typed, not soft wrapping.

Add UserMarkdownRenderer — same styles, width, and Chroma formatter as MarkdownRenderer, plus glamour's WithPreservedNewLines — and use it for user messages only. Assistant output and dialog copy keep the standard renderer, since those really are Markdown and collapsing soft-wrapped paragraphs is the correct rendering there.

The new renderer is memoized per width like its siblings and is cleared by InvalidateMarkdownRendererCache, so a style/palette change does not leave stale user renderers behind.

WithPreservedNewLines only affects soft breaks inside paragraphs and blockquotes: bullet lists, ordered lists, fenced code blocks, tables, and long-line soft wrapping all render byte-identically before and after. Tests cover the reported case, that long lines still wrap, and that those block constructs are unaffected.

Fixes #3502

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

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