docs: add project documentation and update README

- Add `agent.md`: Complete maintenance reference covering project structure, API interactions, data flow, configuration, security, and operational procedures.
- Add `architecture.md`: High-level architecture overview including component map, data flow, design decisions, and security model.
- Update `README.md`: Add Apprise support documentation, manual deployment instructions, updated installation steps (bot admin requirement), and expanded payload mapping and troubleshooting sections.
- Improve `lib/Controller/WebhookController.php`: Enhance `saveBotPassword` validation to check for missing fields and validate the password content.
- Improve `lib/Service/TalkService.php`: Add `validateBotPassword` method for round-trip encryption testing and integrate it into `saveConfig`.
- Update `css/adminSettings.css`: Add styles for the new `.nc-app-frame` wrapper.
- Update `templates/adminSettings.php`: Refactor UI structure to use `.nc-app-frame` and improve layout of settings sections.

css/adminSettings.css

@@ -1,3 +1,15 @@
+.nc-app-frame {
+    border: 1px solid var(--color-border);
+    border-radius: 6px;
+    padding: 1.5em;
+    margin-bottom: 1em;
+}
+.nc-app-frame h2 {
+    margin-top: 0;
+    margin-bottom: 1em;
+    padding-bottom: 0.5em;
+    border-bottom: 1px solid var(--color-border);
+}
 .nc-settings-section {
     margin-bottom: 2em;
     padding: 1em;