Show a shimmer while a thread title regenerates

[ryankhart]

Written by an AI assistant (Claude) for ryankhart.

The problem

• When you regenerate a thread title from the sidebar, the title only fades to 55% opacity until the new one arrives.
• It's easy to miss, so it isn't clear whether anything is happening.

The idea

Reuse the existing live tool-call shine (live-tool-shine) on the title while titleRegeneration is set, so a band sweeps across the old title until the new one lands.

• The old title stays readable while it works.
• It pauses off-screen through observeVisibleAnimation, like the other live shimmers.
• With reduced motion or forced colours it falls back to today's dim.
• No layout change: nothing moves or resizes.

Size

• About 25 lines: one CSS utility in index.css, plus swapping opacity-55 for it in the two places in SidebarThreadRow.tsx (the sidebar row and the thread search result).
• Working version in my fork: ryankhart/t3code@2045c40.

Ask

Would you take a PR for this? The speed and band strength are easy to tune.

Related title ideas: #31024 (shortcut to regenerate) and #31393 (auto-regenerate after each turn).