Move the ntfy alert sink to Settings, shared by all alert rules

Alert rules no longer carry their own ntfy topic/server; Settings now
holds one process-wide ntfy sink (with its own "send test notification"),
and the Alerts panel is left to just define rule conditions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/lib/useSettingsPanel.ts

@@ -153,6 +153,15 @@ export function useSettingsPanel() {
     [guarded],
   )
 
+  const setNtfySink = useCallback(
+    (ntfyTopic: string | null, ntfyServer: string | null) =>
+      guarded(async () => {
+        const next = await updateSettings({ ntfyTopic, ntfyServer })
+        setSettings(next)
+      })(),
+    [guarded],
+  )
+
   const importCssEntry = useCallback(
     (filename: string, content: string) =>
       guarded(async () => {
@@ -230,6 +239,7 @@ export function useSettingsPanel() {
     setColorScheme,
     setCustomTheme,
     setMaxLogs,
+    setNtfySink,
     importCssEntry,
     importThemeEntry,
     linkThemeEntry,