perf(site): fix compiler memoization gap in AgentDetailInput

[mafredri]

Why

In AgentDetailInput, the React Compiler failed to memoize the messages
derivation chain (orderedMessageIDs.map().filter() and
getLatestContextUsage(messages)). Two source patterns prevented the
compiler from grouping them into a single cache guard:

1. A useDashboard() hook call between the messages computation and its
   consumer (getLatestContextUsage). Hooks must be called unconditionally,
   so the compiler cannot wrap non-hook computations in a conditional guard
   when a hook sits between them.

2. An IIFE around the context usage logic, which the compiler lowers into a
   bb0: block with break, fragmenting the dependency chain.

The identical pattern in AgentDetailTimeline (same file) compiled
correctly because its derivation chain has no hook calls or IIFEs between
steps.

Fix

Replace the IIFE with a ternary and reorder the non-hook computation
(rawUsage, latestContextUsage) before the useDashboard() call. This
lets the compiler group the entire chain into one cache guard keyed on
messagesByID and orderedMessageIDs.

Proof

Cache guard simulation test in chatHelpers.test.ts measures guard miss
counts across 10 renders with stable message data:

Guard strategy	Misses (10 renders)
Before (no guard on messages chain)	10/10
After (guard on messagesByID + orderedMessageIDs)	0/10

The test first proves the precondition: getLatestContextUsage always
returns a fresh object reference even when values are identical. Without the
compiler guard, every render produces a new rawUsage ref, which cascades
into a new latestContextUsage, which misses the 34-dep AgentChatInput
JSX guard.

Compiled output confirms the guard structure:

// AFTER: messages + getLatestContextUsage inside guard
if ($[0] !== messagesByID || $[1] !== orderedMessageIDs) {
    const messages = orderedMessageIDs.map(t3).filter(isChatMessage);
    t2 = getLatestContextUsage(messages);
}
// latestContextUsage spread in separate guard
if ($[5] !== compressionThreshold || $[6] !== rawUsage) {
    t3 = rawUsage ? { ...rawUsage, compressionThreshold } : rawUsage;
}

🤖 This PR was created with the help of Coder Agents, and will be reviewed by a human. 🏂🏻

[DanielleMaywood]

I don't think we need to merge the test