Boost page navigation with htmx

Links and GET forms to wiki pages swap the next page into .shell instead
of loading a new document: the server answers boosted requests and
history restores with a partial (title, page actions out-of-band, shell
content). The header stays, browsers without cross-document view
transitions (Firefox) no longer flash between pages, and the swap runs
as a same-document view transition. Files, the editor, internal
endpoints and POST forms still navigate normally.

Back/Forward refetch pages (no snapshots that could predate a move or
delete); the tree rail keeps its scroll position and the content column
returns to where it was. Mutation redirects swap their result in the
same way. Page scripts that assumed a fresh document now also run after
a swap (toc, contents, companion, scroll-sync, search actions) or
register once (fitness).

The e2e suite now also runs in Firefox.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dialogs.go

@@ -15,6 +15,18 @@ func isHX(r *http.Request) bool {
 	return r.Header.Get("HX-Request") == "true"
 }
 
+// layoutTemplate names the template that renders a layout page for r: the
+// whole document, or for boosted navigation and htmx history restores only
+// the "partial" that hx.js swaps into the open page. The editor is never
+// boosted.
+func layoutTemplate(w http.ResponseWriter, r *http.Request, editMode bool) string {
+	w.Header().Set("Vary", "HX-Boosted, HX-History-Restore-Request")
+	if !editMode && (r.Header.Get("HX-Boosted") == "true" || r.Header.Get("HX-History-Restore-Request") == "true") {
+		return "partial"
+	}
+	return "layout"
+}
+
 // redirectAfter sends the client to target once a mutation succeeded. htmx
 // requests get HX-Redirect (assets/hx.js turns it into a history replace, so
 // Back skips the pre-mutation page); plain form posts get a 303.