chore(web): extract App.tsx's leaf hooks (#2128)

Phase 1 of #2126. Six hooks move out of `clients/web/src/App.tsx`, taking
its component body from 4,891 to 4,267 lines with no behavior change:

- useThemeToggle       — the light/dark toggle
- useTabUiState        — the nine per-screen `ui` objects, the active tab,
                         the pinned-Protocol ids, and the session reset
- useInspectorStores   — the twelve state managers as one create/destroy
                         lifecycle, their `core/react` hooks, and the
                         pagination-mode selection
- useExportActions     — the eleven Clear / Export / Replay handlers
- useProgressToasts    — `notifications/progress` toasts
- useTaskToasts        — task-status toasts, the cancel confirmation, the
                         taskId -> progress map, and the in-flight
                         tool-call taskId ref

Holding the stores in one slot collapses `setupClientForServer`'s and
`onConfirmRemove`'s dependency arrays from twelve entries to two stable
callbacks; `destroyStores` reads the live set through a ref mirror rather
than the state it also writes.

All six hooks are at 100% on all four coverage dimensions, no `v8 ignore`
needed. Also refreshes the now-stale "~4.5k-line" description of App.tsx
in AGENTS.md and the coverage `include` comment.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/hooks/useThemeToggle.ts

@@ -0,0 +1,19 @@
+import { useCallback } from "react";
+import { useComputedColorScheme, useMantineColorScheme } from "@mantine/core";
+
+/**
+ * Light/dark toggle for the view header.
+ *
+ * Reads `useComputedColorScheme` rather than the stored preference, so "auto"
+ * resolves to the scheme the user is actually looking at — which is what makes
+ * a single toggle button flip the right way under the system default.
+ */
+export function useThemeToggle(): { onToggleTheme: () => void } {
+  const { setColorScheme } = useMantineColorScheme();
+  const computedColorScheme = useComputedColorScheme("light");
+  const isDark = computedColorScheme === "dark";
+  const onToggleTheme = useCallback(() => {
+    setColorScheme(isDark ? "light" : "dark");
+  }, [isDark, setColorScheme]);
+  return { onToggleTheme };
+}