refactor(ui): modularize instance shell

Split InstanceShell2 into focused shell modules (drawer chrome/resize, session context/cache, sidebar, right panel tabs/components) to improve maintainability while preserving behavior.

packages/ui/src/components/instance/shell/useDrawerHostMeasure.ts

@@ -0,0 +1,65 @@
+import { createEffect, createSignal, type Accessor } from "solid-js"
+
+type DrawerHostMeasure = {
+  setDrawerHost: (element: HTMLElement) => void
+  drawerContainer: () => HTMLElement | undefined
+  measureDrawerHost: () => void
+  floatingTopPx: () => string
+  floatingHeight: () => string
+}
+
+export function useDrawerHostMeasure(tabBarOffset: Accessor<number>): DrawerHostMeasure {
+  const [drawerHost, setDrawerHost] = createSignal<HTMLElement | null>(null)
+  const [floatingDrawerTop, setFloatingDrawerTop] = createSignal(0)
+  const [floatingDrawerHeight, setFloatingDrawerHeight] = createSignal(0)
+
+  const storeDrawerHost = (element: HTMLElement) => {
+    setDrawerHost(element)
+  }
+
+  const measureDrawerHost = () => {
+    if (typeof window === "undefined") return
+    const host = drawerHost()
+    if (!host) return
+    const rect = host.getBoundingClientRect()
+    setFloatingDrawerTop(rect.top)
+    setFloatingDrawerHeight(Math.max(0, rect.height))
+  }
+
+  createEffect(() => {
+    tabBarOffset()
+    if (typeof window === "undefined") return
+    requestAnimationFrame(() => measureDrawerHost())
+  })
+
+  const drawerContainer = () => {
+    const host = drawerHost()
+    if (host) return host
+    if (typeof document !== "undefined") {
+      return document.body
+    }
+    return undefined
+  }
+
+  const fallbackDrawerTop = () => tabBarOffset()
+  const floatingTop = () => {
+    const measured = floatingDrawerTop()
+    if (measured > 0) return measured
+    return fallbackDrawerTop()
+  }
+
+  const floatingTopPx = () => `${floatingTop()}px`
+  const floatingHeight = () => {
+    const measured = floatingDrawerHeight()
+    if (measured > 0) return `${measured}px`
+    return `calc(100% - ${floatingTop()}px)`
+  }
+
+  return {
+    setDrawerHost: storeDrawerHost,
+    drawerContainer,
+    measureDrawerHost,
+    floatingTopPx,
+    floatingHeight,
+  }
+}