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:

booth/templates/view.html

@@ -11,8 +11,20 @@
     </span>
     <a class="vbtn" href="{{ file_url }}" download title="download {{ file }}">⬇</a>
   </div>
+  {% if prev_url %}<a class="vnav vprev" href="?f={{ prev_url }}" title="previous (←)" aria-label="previous image">‹</a>{% endif %}
+  {% if next_url %}<a class="vnav vnext" href="?f={{ next_url }}" title="next (→)" aria-label="next image">›</a>{% endif %}
   <div class="vstage fit" id="vstage"><img id="vimg" src="{{ file_url }}" alt="{{ file }}"></div>
 </div>
+<style>
+  .vnav{position:fixed;top:50%;transform:translateY(-50%);z-index:40;display:flex;
+    align-items:center;justify-content:center;width:2.6rem;height:3.4rem;font-size:2rem;
+    line-height:1;text-decoration:none;color:var(--fg-1);background:rgba(20,23,32,.55);
+    border:1px solid rgba(255,255,255,.10);border-radius:10px;margin:0 .5rem;user-select:none;
+    -webkit-backdrop-filter:blur(4px);backdrop-filter:blur(4px);transition:background .15s,border-color .15s}
+  .vnav:hover{background:rgba(28,33,46,.92);border-color:var(--aus-bright-cyan,#42dcd1)}
+  .vprev{left:0}.vnext{right:0}
+  @media print{.vnav{display:none}}
+</style>
 <script>
   (function () {
     var img = document.getElementById('vimg');
@@ -21,6 +33,8 @@
     var bFit = document.getElementById('btn-fit');
     var bOne = document.getElementById('btn-one');
     var BACK = {{ ('/b/' ~ name_url ~ '/')|tojson }};
+    var PREV = {{ (('?f=' ~ prev_url) if prev_url else '')|tojson }};
+    var NEXT = {{ (('?f=' ~ next_url) if next_url else '')|tojson }};
 
     function setMode(mode) {
       var fit = mode === 'fit';
@@ -51,6 +65,8 @@
     if (img.complete) evaluate();
     document.addEventListener('keydown', function (e) {
       if (e.key === 'Escape') window.location.href = BACK;
+      else if (e.key === 'ArrowLeft' && PREV) window.location.href = PREV;
+      else if (e.key === 'ArrowRight' && NEXT) window.location.href = NEXT;
     });
   })();
 </script>