Merge gitea/develop into develop

Reconcile the diverged develop (13 ahead / 20 behind) with gitea/develop.

Conflict resolution — html-embed: keep the local sandboxed-iframe model
(opaque-origin srcdoc, no role-gating) and supersede gitea's same-origin
strip/kill-switch hardening (#26/#28/#29/#30). The 4 conflicted html-embed
source files resolve to the local version; the 3 strip-era spec files stay
deleted. The strip apparatus (stripDisallowedHtmlEmbedNodes,
collectHtmlEmbedSources, canAuthorHtmlEmbed, htmlEmbedAllowed) is fully gone.

Integrate gitea's page-templates / page-embed work (#31-#40) cleanly.

Fix an auto-merge arity mismatch: two new gitea page-template specs
constructed TransclusionService with the pre-sandbox 11-arg signature; drop
the trailing workspaceRepo argument to match the reduced 10-arg constructor.

Verified: server + client tsc --noEmit clean; jest (html-embed + transclusion)
14 suites / 119 tests passing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/public/locales/en-US/translation.json

@@ -183,6 +183,7 @@
   "Successfully imported": "Successfully imported",
   "Successfully restored": "Successfully restored",
   "System settings": "System settings",
+  "Template": "Template",
   "Templates": "Templates",
   "Theme": "Theme",
   "To change your email, you have to enter your password and new email.": "To change your email, you have to enter your password and new email.",
@@ -473,6 +474,7 @@
   "Make sub-pages public too": "Make sub-pages public too",
   "Allow search engines to index page": "Allow search engines to index page",
   "Open page": "Open page",
+  "Open source page": "Open source page",
   "Page": "Page",
   "Delete public share link": "Delete public share link",
   "Delete share": "Delete share",