feat(web): Sessions, Rate-Limiting, Security-Header, Templates und eingebettete Assets

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NBHF4R9EAejDJUMdwr6C68

internal/web/templates/error.html

@@ -0,0 +1,8 @@
+{{define "content"}}
+<h1>{{.Data.Title}}</h1>
+<div class="card error">
+  <p>{{.Data.Message}}</p>
+  {{if .Data.ShowLoginLink}}<p><a href="/login">{{t "login_submit"}}</a></p>{{end}}
+  {{if .Data.ShowOverviewLink}}<p><a href="/">{{t "back_to_overview"}}</a></p>{{end}}
+</div>
+{{end}}