perf(desktop): isolate streaming re-renders & cut layout thrash

During a token stream $messages is replaced ~30x/s. Subscribing the whole
chat view to it re-rendered the composer, runtime boundary, and every
message on every delta.

- Derive coarse facts (empty thread? tail is user?) via nanostores
  `computed` atoms so per-token flushes don't re-render their consumers.
- Move the $messages subscription + runtime wiring into a dedicated
  ChatRuntimeBoundary; the composer reads $messages imperatively.
- Drive message rows off stable useAuiState selectors and a lazy
  getMessageText getter instead of eagerly materialized text.
- Feed ResizeObserver entry sizes into measureClamp / FadeText and dedupe
  the style writes, killing the read-write-read reflow cascade.

apps/desktop/src/hooks/use-resize-observer.ts

@@ -1,17 +1,26 @@
 import { type RefObject, useLayoutEffect, useRef } from 'react'
 
-export function useResizeObserver(onResize: () => void, ...refs: readonly RefObject<Element | null>[]) {
+/**
+ * Observe element resizes. The callback receives the ResizeObserver entries
+ * (empty on the initial synchronous call and in non-RO environments) so
+ * callers can read the observed size off the entry instead of forcing a
+ * fresh layout read.
+ */
+export function useResizeObserver(
+  onResize: (entries: readonly ResizeObserverEntry[]) => void,
+  ...refs: readonly RefObject<Element | null>[]
+) {
   const refsRef = useRef(refs)
   refsRef.current = refs
 
   useLayoutEffect(() => {
     if (typeof ResizeObserver === 'undefined') {
-      onResize()
+      onResize([])
 
       return
     }
 
-    const observer = new ResizeObserver(() => onResize())
+    const observer = new ResizeObserver(entries => onResize(entries))
     let observed = false
 
     for (const ref of refsRef.current) {
@@ -31,7 +40,7 @@ export function useResizeObserver(onResize: () => void, ...refs: readonly RefObj
       return
     }
 
-    onResize()
+    onResize([])
 
     return () => observer.disconnect()
   }, [onResize])