Tools: Frame tool (#2914) (#2915)

The Frame tool (K, its own toolbar slot after the Crop group) draws a
rectangular or elliptical frame; the options bar picks the shape, Shift
constrains to a square or circle and Alt draws from the centre. A frame
drawn over the selected pixel layer's content takes that layer in;
otherwise it is an empty frame.

Frames are built as Frame from Layers builds them: a group clipped by a
layer mask (rectangular or anti-aliased elliptical). The new engine
command layer.new.frame does it in one Session::edit, so one undo step.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

assets/icons/frame-placeholder.svg

@@ -0,0 +1,15 @@
+<svg
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <!-- PhotoCraft's own Frame tool glyph (not from an icon set): an empty placeholder frame, crossed corner to corner. -->
+  <rect x="3" y="4" width="18" height="16" rx="1" />
+  <path d="M6.5 7.5l11 9M17.5 7.5l-11 9" />
+</svg>