docs: add Pushover and improve push notification documentation (#27)

- Add Pushover to supported services list in UI
- Add Gotify to supported services list
- Add URL placeholder with examples (ntfy, pushover)
- Add link to shoutrrr.dev for all available services
- Change input type from 'url' to 'text' (shoutrrr URLs aren't HTTP URLs)
- Add comprehensive Push Notifications section to README
- Include URL examples for ntfy, Pushover, Gotify, Discord, Telegram

Closes feature request for Pushover support.

frontend/src/i18n/de.json

@@ -173,7 +173,9 @@
     },
     "push": {
       "url": "URL",
-      "supports": "Unterstützt ntfy, Discord, Telegram, Slack"
+      "urlPlaceholder": "ntfy://topic oder pushover://:token@userkey/",
+      "supports": "Unterstützt ntfy, Pushover, Gotify, Discord, Telegram, Slack & mehr",
+      "docsLink": "Siehe shoutrrr.dev für alle Services"
     },
     "schedule": {
       "title": "Erinnerungsplan",

frontend/src/i18n/en.json

@@ -175,7 +175,9 @@
     },
     "push": {
       "url": "URL",
-      "supports": "Supports ntfy, Discord, Telegram, Slack"
+      "urlPlaceholder": "ntfy://topic or pushover://:token@userkey/",
+      "supports": "Supports ntfy, Pushover, Gotify, Discord, Telegram, Slack & more",
+      "docsLink": "See shoutrrr.dev for all services"
     },
     "schedule": {
       "title": "Reminder Schedule",