feat(booth): prev/next arrows in the image viewer

Zooming an image now shows ‹ / › arrows at the left/right edges that step
to the previous/next image in the booth (gallery sorted-rel order), wrapping
around, plus keyboard ←/→. Arrows are hidden when a booth has a single image.
booth_view_file computes neighbors via a new booth_image_names() helper and
passes prev_url/next_url to view.html. 3 new tests, suite 47 passing;
deployed + verified live on nh3-dev :8090.

booth/app.py

@@ -77,6 +77,15 @@ def render_doc(text: str, kind: str) -> tuple[str, bool]:
     return text, False
 
 
+def booth_image_names(child: Path) -> list[str]:
+    """Image files in a booth, in gallery (sorted-rel) order — for viewer prev/next."""
+    return sorted(
+        p.relative_to(child).as_posix()
+        for p in child.rglob("*")
+        if p.is_file() and not p.name.startswith(".") and classify(p.name) == "image"
+    )
+
+
 def classify(name: str) -> str:
     """image | video | audio | other, by extension."""
     ext = Path(name).suffix.lower()
@@ -525,7 +534,16 @@ def create_app(
         file_url = quote(f, safe="/")
         common = {**base_ctx, "name": name, "name_url": quote(name, safe=""), "file": f, "file_url": file_url}
         if classify(target.name) == "image":
-            return templates.TemplateResponse(request, "view.html", common)
+            # prev/next image nav (wraps around; only when >1 image in the booth)
+            names = booth_image_names(booth)
+            prev_url = next_url = None
+            if f in names and len(names) > 1:
+                i = names.index(f)
+                prev_url = quote(names[(i - 1) % len(names)], safe="/")
+                next_url = quote(names[(i + 1) % len(names)], safe="/")
+            return templates.TemplateResponse(
+                request, "view.html", {**common, "prev_url": prev_url, "next_url": next_url}
+            )
         # .md renders, .txt/.log show as text — viewable in-booth, no download
         dk = doc_kind(target.name)
         if dk: