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.test.tsx

@@ -0,0 +1,42 @@
+import { describe, expect, it } from "vitest";
+import { act } from "react";
+import { useComputedColorScheme } from "@mantine/core";
+import { renderWithMantine } from "../test/renderWithMantine";
+import { useThemeToggle } from "./useThemeToggle";
+
+/**
+ * Renders the hook alongside the resolved scheme, so each assertion reads what
+ * the user would actually be looking at rather than what was stored.
+ */
+function harness(colorScheme?: "light" | "dark") {
+  let toggle: (() => void) | undefined;
+  let scheme = "";
+  function Probe() {
+    toggle = useThemeToggle().onToggleTheme;
+    scheme = useComputedColorScheme("light");
+    return null;
+  }
+  renderWithMantine(<Probe />, colorScheme ? { colorScheme } : undefined);
+  return {
+    scheme: () => scheme,
+    toggle: () => {
+      act(() => toggle?.());
+    },
+  };
+}
+
+describe("useThemeToggle", () => {
+  it("switches a light scheme to dark", () => {
+    const h = harness();
+    expect(h.scheme()).toBe("light");
+    h.toggle();
+    expect(h.scheme()).toBe("dark");
+  });
+
+  it("switches a dark scheme back to light", () => {
+    const h = harness("dark");
+    expect(h.scheme()).toBe("dark");
+    h.toggle();
+    expect(h.scheme()).toBe("light");
+  });
+});