fix(notifications): send content and text so Discord and Slack webhooks deliver

The generic webhook body carried title, message, eventType and priority
only. Discord refuses a body without content (400, code 50006 "Cannot
send an empty message") and a Slack incoming webhook one without text,
so the test button and every reminder failed against the targets the
settings card names.

The generic body now appends content (cut to Discord's 2000 characters)
and text, both the masked title and message on two lines, plus
allowed_mentions with an empty parse list so Discord resolves no
mentions. The four original fields keep their values and order for
Home Assistant and n8n rules. The Gotify body is unchanged. The card
names what works and shows a hint for the generic format.

src/components/settings/integrations/notification-channels-panel.tsx

@@ -71,8 +71,8 @@ export function NotificationChannelsPanel() {
           <WebPushCard />
         </div>
       )}
-      {/* v1.17.1 — generic outbound webhook (Gotify / Discord / Slack /
-          Matrix / Home Assistant in one channel). */}
+      {/* v1.17.1 — generic outbound webhook (Discord / Slack / Gotify /
+          Home Assistant / n8n in one channel). */}
       <div id="webhook" className="scroll-mt-28">
         <WebhookCard isAuthenticated={isAuthenticated} />
       </div>

src/components/settings/webhook-card.tsx

@@ -209,11 +209,11 @@ export function WebhookCard({ isAuthenticated }: { isAuthenticated: boolean }) {
                   {t("settings.webhookFormatGotify")}
                 </option>
               </NativeSelect>
-              {format === "gotify" && (
-                <p className="text-muted-foreground text-xs">
-                  {t("settings.webhookFormatGotifyHint")}
-                </p>
-              )}
+              <p className="text-muted-foreground text-xs">
+                {format === "gotify"
+                  ? t("settings.webhookFormatGotifyHint")
+                  : t("settings.webhookFormatGenericHint")}
+              </p>
             </div>
             <div className="space-y-2">
               <Label htmlFor="webhook-header-name">