fix: pass notification settings from create form to service (#160)

## Summary

`HandleAppCreate` was not reading `docker_network`, `ntfy_topic`, or `slack_webhook` form values from the create app form. These fields were silently dropped during app creation, even though:
- `app_new.html` had the form fields
- `CreateAppInput` had the corresponding struct fields
- `CreateApp` already handled them correctly

The edit/update flow was unaffected — the bug was exclusively in the create path.

## Changes

- Read `docker_network`, `ntfy_topic`, `slack_webhook` form values in `HandleAppCreate`
- Pass them to `CreateAppInput`
- Include them in template re-render data (preserves values on validation errors)

closes #157

<!-- session: agent:sdlc-manager:subagent:1fb3582d-1eff-4309-b166-df5046a1b885 -->

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #160
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/handlers/app.go

@@ -54,12 +54,18 @@ func (h *Handlers) HandleAppCreate() http.HandlerFunc { //nolint:funlen // valid
 		repoURL := request.FormValue("repo_url")
 		branch := request.FormValue("branch")
 		dockerfilePath := request.FormValue("dockerfile_path")
+		dockerNetwork := request.FormValue("docker_network")
+		ntfyTopic := request.FormValue("ntfy_topic")
+		slackWebhook := request.FormValue("slack_webhook")
 
 		data := h.addGlobals(map[string]any{
 			"Name":           name,
 			"RepoURL":        repoURL,
 			"Branch":         branch,
 			"DockerfilePath": dockerfilePath,
+			"DockerNetwork":  dockerNetwork,
+			"NtfyTopic":      ntfyTopic,
+			"SlackWebhook":   slackWebhook,
 		}, request)
 
 		if name == "" || repoURL == "" {
@@ -100,6 +106,9 @@ func (h *Handlers) HandleAppCreate() http.HandlerFunc { //nolint:funlen // valid
 				RepoURL:        repoURL,
 				Branch:         branch,
 				DockerfilePath: dockerfilePath,
+				DockerNetwork:  dockerNetwork,
+				NtfyTopic:      ntfyTopic,
+				SlackWebhook:   slackWebhook,
 			},
 		)
 		if createErr != nil {