feat: replace Bootstrap with custom CSS and add profile page

- Add self-hosted style.css replacing Bootstrap CDN dependency
- Add profile.html with tabbed view (account info, emails/aliases, API token)
- Refactor login handler to pass structured data to template instead of building HTML strings
- Add brand-name and brand-logo flags/env vars for UI customization
- Update CSP to allow brand logo domain and remove CDN references
- Update all templates to pass template vars to header/footer and use new CSS classes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/message.html

@@ -1,5 +1,7 @@
-{{template "header"}}
-  {{if .message}}<div class="alert alert-success">{{.message}}</div>{{end}}
-  {{if .error}}<div class="alert alert-danger">{{.error}}</div>{{end}}
-  {{if .details}}<p>{{.details}}</p>{{end}}
-{{template "footer"}}
+{{template "header" .}}
+  <div class="message-page">
+    {{if .message}}<div class="alert alert-success">{{.message}}</div>{{end}}
+    {{if .error}}<div class="alert alert-error">{{.error}}</div>{{end}}
+    {{if .details}}<p class="details-text">{{.details}}</p>{{end}}
+  </div>
+{{template "footer" .}}