fix(desktop): resolve composer cursor drift on multi-line input

[wesbillman]

Problem

When typing multi-line text in the message composer, the cursor drifts upward from the actual text position. By the 3rd line, the cursor is ~12px above where the text appears, making it difficult to see what you're typing.

Root Cause

The base Textarea component (shared/ui/textarea.tsx) includes md:text-sm in its default classes. At the md breakpoint, md:text-sm sets both font-size: 0.875rem and line-height: 1.25rem (20px). The composer overrides with leading-6 (24px line-height), but the responsive md:text-sm has higher CSS specificity and wins at desktop widths.

Meanwhile, the ComposerMentionOverlay (a plain <div>) uses text-sm leading-6 without any responsive override, so it keeps line-height: 24px at all breakpoints.

This mismatch — textarea at 20px vs overlay at 24px — causes the caret to drift 4px per line away from the overlay's visible text.

Fix

Add md:leading-6 to the Textarea className in MessageComposer.tsx so the line-height override wins at the md breakpoint too. This ensures both the textarea and overlay use a consistent 24px line-height at all breakpoints.

Follow-up (separate PR)

• Extract shared text classes into a constant for explicit textarea ↔ overlay coupling
• Replace DOM-read lineHeightRef with a constant (24px at all breakpoints now)
• Add defensive md:leading-6 to the overlay for symmetry