Optimistic pending: apply loading state instantly on click

[adnaan]

Problem

When a user clicks a button that triggers Async work, the loading indicator ({{.lvt.Pending}}) only appears after the server round-trip completes and sends back the first WS frame (the "pending render"). On slow connections this creates a perceptible lag where nothing visually changes after the click.

The current flow:

1. User clicks → client sends WS action message
2. Server receives, runs the action handler, sets Pending=true, re-renders
3. Server sends pending-render WS frame (frame Add comprehensive file upload support with S3 integration #1)
4. Only now does the client show the loading state
5. Async work completes → server sends completion-render WS frame (frame Add Claude Code GitHub Workflow #2)

Proposal

The client should optimistically apply a CSS class or data attribute (e.g., data-lvt-pending or a class on the triggering element/form) immediately on click, before the WS round-trip. The server's pending-render frame then confirms the state; the completion-render frame clears it.

This makes the loading indicator appear instantly regardless of network latency.

Design questions

• Scope: Should the pending indicator be per-form (the <form> ancestor of the clicked button), per-element (the clicked <button> itself), or per-page (a global pending state)?
• Attribute vs class: data-lvt-pending attribute lets CSS target [data-lvt-pending]; a class like lvt-pending is also viable.
• Clearing: The attribute/class should be cleared when the server's completion render arrives (frame Add Claude Code GitHub Workflow #2). If the server never responds (connection drop), the client's reconnect/error handling should also clear it.

Context

• livetemplate.Async[S, R] and {{.lvt.Pending}} shipped in livetemplate v0.22.0
• Server-side Pending is per-render (true on the render that registered Async work, false on all others)
• See livetemplate/livetemplate#481 for the original boilerplate-reduction investigation
• Example app: docs/examples/greet-async/