fix(chat): make smooth conversation follow opt-in

[AprilNEA]

What changed

• make conversation switches and pinned content growth jump directly to the bottom by default
• add a persisted Settings → Appearance → Smooth conversation follow toggle, defaulted off; Reduce motion always forces instant positioning
• keep the scroll pinned synchronously while virtua settles its measured row sizes
• extend the real Chromium smoke flow to cover bounded mounts, settled bottom position after two animation frames, and the new setting

Root cause

Conversation used initial="instant" but retained resize="smooth". use-stick-to-bottom applies initial only to the first observed height; switching sessions remounts the conversation, and virtua then reports additional positive resizes while replacing estimates with measured rows. Those later resizes entered the smooth spring path, so the newly selected conversation visibly scrolled even though its initial positioning was instant. The dependency's instant path also waits for requestAnimationFrame, which left a visible intermediate position during measurement settling.

Impact

Switching conversations now opens directly at the latest message without a scroll animation. Users who prefer animated following can opt in from Appearance settings.

Verification

• pnpm check:ci
• pnpm test — 302 files passed, 3 skipped; 2,435 tests passed, 5 skipped
• pnpm -F @linkcode/webview e2e:browser
• red/green animation check: leaving smooth follow enabled reproduced the regression at 76 px above bottom after two frames; the default-off implementation settles within the 2 px tolerance
• virtualization mutation check: removing itemSize={300} mounts all 48 fixture turns and fails the < 10 bound; the restored implementation mounts 6 and passes

Tracks CODE-259.

[linear-code]

CODE-259