feat(web): Webhooks page + ALERTS nav group

New /webhooks admin page with table-based subscription management:
- CREATE WEBHOOK (inline form row — no modal) with simple-event
  checkboxes (AttackerDetail / DeckyStatus / SystemStatus) that
  expand to bus-topic patterns server-side, and an advanced-mode
  textarea for raw NATS-style patterns.
- Bulk-select + DELETE SELECTED with two-click arm pattern.
- Per-row test-ping (zap), pencil edit, and delete actions.
- Last-fired timestamp column.
- Yellow banner surfacing insecure_url warnings (WH-03): http:// is
  allowed but flagged so operators see it on every page load.
- Post-create secret modal — the secret is shown exactly once with
  a COPY button and a clear "won't see this again" notice.

Sidebar nav regrouped: /live-logs and /webhooks now live under a new
ALERTS NavGroup (Bell icon). The alertCount badge rides the Live
Logs sub-item. Command palette gains a "Webhooks" GO TO entry with
the `G W` chord.

Side-fix: useFocusSearch.ts was failing the build under
verbatimModuleSyntax (pre-existing, unrelated). Split the React
import to satisfy tsc; no behavioural change.

decnet_web/src/hooks/useFocusSearch.ts

@@ -1,4 +1,5 @@
-import { useEffect, RefObject } from 'react';
+import { useEffect } from 'react';
+import type { RefObject } from 'react';
 
 /**
  * Focus the given input when the global `decnet:focus-search` event fires