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.

services/booth/tests/test_booth.py

@@ -493,3 +493,38 @@ def test_gallery_links_docs_to_view(client):
     assert "view?f=readme.md" in page.text   # md -> viewer
     assert "view?f=notes.txt" in page.text   # txt -> viewer
     assert 'href="readme.md" download' not in page.text   # not a forced download
+
+
+# ---- image viewer prev/next nav ---------------------------------------------
+
+
+def test_view_image_prev_next_nav(client):
+    c, data = client
+    d = data / "run1"; d.mkdir()
+    for n in ("a.png", "b.png", "c.png"):
+        _touch(d / n)
+    r = c.get("/b/run1/view", params={"f": "b.png"})   # middle -> prev=a, next=c
+    assert r.status_code == 200
+    assert 'class="vnav vprev" href="?f=a.png"' in r.text
+    assert 'class="vnav vnext" href="?f=c.png"' in r.text
+
+
+def test_view_image_nav_wraps(client):
+    c, data = client
+    d = data / "run1"; d.mkdir()
+    for n in ("a.png", "b.png", "c.png"):
+        _touch(d / n)
+    first = c.get("/b/run1/view", params={"f": "a.png"}).text
+    assert 'vprev" href="?f=c.png"' in first and 'vnext" href="?f=b.png"' in first  # first wraps prev->last
+    last = c.get("/b/run1/view", params={"f": "c.png"}).text
+    assert 'vnext" href="?f=a.png"' in last and 'vprev" href="?f=b.png"' in last    # last wraps next->first
+
+
+def test_view_single_image_no_nav(client):
+    c, data = client
+    d = data / "run1"; d.mkdir()
+    _touch(d / "only.png")
+    r = c.get("/b/run1/view", params={"f": "only.png"})
+    assert r.status_code == 200
+    # no arrow anchors with a single image (the .vnav CSS rule is always present)
+    assert 'class="vnav vprev"' not in r.text and 'class="vnav vnext"' not in r.text