feat(message): collapse very long messages behind an expand toggle

[Th-Underscore]

Description

Very long messages (past ~30 lines) no longer dominate the timeline: the body collapses behind an expand/collapse toggle. Collapsed bodies are clamped with a fade-out hint, and expanding restores the full height inline without disturbing scroll position more than necessary.

Purely cosmetic timeline behavior - message content is untouched, rendering only.

Tested on the desktop app, Ubuntu 24.04.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - .changeset
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

I designed and directed the feature and tested it in the GUI; the code itself was AI-generated.

A self-contained LongMessageCollapse component measures rendered height (hidden measurement row, thresholds above 30 lines) and swaps between clamped and full render, wrapping the body in display: inline-block while the content fits, so the (edited) indicator stays beside the content rather than being pushed below it. It hooks into the RenderMessageContent body funnel so every renderer picks it up uniformly.

[Th-Underscore]

Would be willing to make it a little more prominent, like a fading tail above "Show More".