feat(web): add ? cheatsheet and / focus-search hotkeys

- New ShortcutsHelp modal enumerates global, nav G-chord and palette
  bindings; openable via ? (Shift+/) or the command palette.
- / dispatches a global decnet:focus-search event; Attackers, Bounty
  and LiveLogs listen and focus their in-page search inputs (pages
  without a local search are skipped per plan).
- Respects the existing editable-element guard and Alt+K palette
  toggle; no rebinds to prior shortcuts.

decnet_web/src/hooks/useFocusSearch.ts

@@ -0,0 +1,18 @@
+import { useEffect, RefObject } from 'react';
+
+/**
+ * Focus the given input when the global `decnet:focus-search` event fires
+ * (dispatched by the `/` hotkey in useGlobalHotkeys).
+ */
+export function useFocusSearch(ref: RefObject<HTMLInputElement | null>): void {
+  useEffect(() => {
+    const handler = () => {
+      const el = ref.current;
+      if (!el) return;
+      el.focus();
+      try { el.select(); } catch { /* ignore */ }
+    };
+    window.addEventListener('decnet:focus-search', handler);
+    return () => window.removeEventListener('decnet:focus-search', handler);
+  }, [ref]);
+}