chore(webui): remove /preview/* routes + rename nav_consolidated → nav_main

Epic #1624 constraint: /preview/* routes removed once consolidated.
- Delete templates/preview/ (6 template files)
- Delete preview.go, preview_services.go, preview_stubs.go, preview_stubs_test.go, preview_test.go
- Add preview_stub.go with no-op registerPreview for webui_preview builds
- Rename nav_consolidated.html → nav_main.html (proper noun)
- Build passes both default and webui_preview modes

Author: nextlevelshit

internal/webui/preview.go

@@ -0,0 +1,7 @@
+//go:build webui_preview
+
+package webui
+
+// registerPreview is a no-op stub when the webui_preview build tag is active.
+// Real implementations are added in Phase B/C.
+func registerPreview(r *FeatureRegistry) {}