ux(ocr): full-bleed modal on mobile, buttons hug screen edges

@media (max-width: 640px):
- backdrop loses its 1 rem inset; modal goes 100vw × 100vh with
  no border-radius
- crop modal body keeps ~20 px horizontal padding so cropper
  corner handles stay safely outside Android's edge-swipe
  back-gesture zone (the corner DRAG is what triggers it; a tap
  on a button at the edge doesn't)
- camera viewfinder body has zero padding — video fills the
  whole space
- footer has minimal 0.5 rem padding so the rotate / invert /
  cancel / check buttons hug the screen edges
- iOS notch / home-indicator insets honoured via
  env(safe-area-inset-*)

Doesn't change the camera-confirm behaviour — that's the OS-camera
fallback path that fires when getUserMedia is denied for not being
a secure context (i.e. http://lan-ip:8000 from the phone, not
HTTPS or localhost).

static/index.html

@@ -299,6 +299,41 @@
       display: block;
     }
 
+    /* On mobile: full-bleed modal, buttons hug the screen edges. Body padding
+       still ~20 px so cropper corner handles stay outside Android's edge-swipe
+       gesture zone — tapping a button doesn't trigger it, but dragging into
+       the very edge does. */
+    @media (max-width: 640px) {
+      .modal-backdrop { padding: 0; }
+      .modal {
+        width: 100vw;
+        max-width: 100vw;
+        height: 100vh;
+        max-height: 100vh;
+        border-radius: 0;
+      }
+      .modal-header {
+        padding-top: max(0.75rem, env(safe-area-inset-top, 0px));
+      }
+      .modal-body {
+        padding:
+          1rem
+          max(1.25rem, env(safe-area-inset-right, 0px))
+          1rem
+          max(1.25rem, env(safe-area-inset-left, 0px));
+        max-height: none;
+      }
+      .cam-body { padding: 0 !important; }
+      .cam-body video { max-height: none; }
+      .modal-footer {
+        padding:
+          0.5rem
+          max(0.5rem, env(safe-area-inset-right, 0px))
+          max(0.5rem, env(safe-area-inset-bottom, 0px))
+          max(0.5rem, env(safe-area-inset-left, 0px));
+      }
+    }
+
     /* CodeMirror editor host — matches .output styling so input & output align */
     .cm-editor {
       min-height: 180px;