feat(booth): image viewer page with Fit/1:1, download, Esc-back (v0.1.2)

Clicking a gallery image now opens a dedicated viewer instead of dumping you on
the raw file.

- GET /b/<name>/view?f=<img> — full-viewport viewer (registered before the file
  catch-all so /view wins; non-image f 307-redirects to the raw file, traversal
  and missing f 404).
- Fit (downscale-only) / 1:1 (natural pixels, scroll-to-pan) toggle that only
  appears when the image is larger than the viewport — when it already fits,
  Fit ≡ 1:1 so the toggle is hidden. Re-evaluates on resize.
- Download button + ✕/Esc back to the gallery. Australis-themed, progressive
  JS (degrades to fit-only, no-JS still shows the image + download + back).
- 5 new tests (34 total, all green); verified Fit/1:1/hidden-toggle states in a
  real browser.

booth/templates/view.html

@@ -0,0 +1,57 @@
+{% extends "base.html" %}
+{% block title %}{{ file }} · {{ name }} · The Booth{% endblock %}
+{% block content %}
+<div class="viewer">
+  <div class="vbar">
+    <a class="vbtn vx" href="/b/{{ name_url }}/" title="back to gallery (Esc)">✕</a>
+    <span class="vname">{{ file }}</span>
+    <span class="vspacer"></span>
+    <span class="vtoggle" id="vtoggle" style="display:none">
+      <button type="button" class="vseg on" id="btn-fit">Fit</button><button type="button" class="vseg" id="btn-one">1:1</button>
+    </span>
+    <a class="vbtn" href="{{ file_url }}" download title="download {{ file }}">⬇</a>
+  </div>
+  <div class="vstage fit" id="vstage"><img id="vimg" src="{{ file_url }}" alt="{{ file }}"></div>
+</div>
+<script>
+  (function () {
+    var img = document.getElementById('vimg');
+    var stage = document.getElementById('vstage');
+    var toggle = document.getElementById('vtoggle');
+    var bFit = document.getElementById('btn-fit');
+    var bOne = document.getElementById('btn-one');
+    var BACK = {{ ('/b/' ~ name_url ~ '/')|tojson }};
+
+    function setMode(mode) {
+      var fit = mode === 'fit';
+      stage.classList.toggle('fit', fit);
+      stage.classList.toggle('one', !fit);
+      bFit.classList.toggle('on', fit);
+      bOne.classList.toggle('on', !fit);
+    }
+    // "fits" == the image at natural size already sits inside the stage, so Fit
+    // and 1:1 would render identically — in that case we hide the toggle entirely.
+    function fits() {
+      return img.naturalWidth <= stage.clientWidth && img.naturalHeight <= stage.clientHeight;
+    }
+    function evaluate() {
+      if (!img.naturalWidth) return;
+      if (fits()) {
+        toggle.style.display = 'none';
+        setMode('fit');
+      } else {
+        toggle.style.display = 'inline-flex';
+        if (!stage.classList.contains('one')) setMode('fit');
+      }
+    }
+    bFit.addEventListener('click', function () { setMode('fit'); });
+    bOne.addEventListener('click', function () { setMode('one'); });
+    img.addEventListener('load', evaluate);
+    window.addEventListener('resize', evaluate);
+    if (img.complete) evaluate();
+    document.addEventListener('keydown', function (e) {
+      if (e.key === 'Escape') window.location.href = BACK;
+    });
+  })();
+</script>
+{% endblock %}