Design tab: give every drawer entry a distinct icon

Six of the surface entries, both Thicken variants, Rib, Axis, Coord Sys,
Mate and Delete Face all reused a sibling's glyph, so a drawer opened as a
column of identical faces and the card that opened rarely matched the entry
clicked. Fixed both halves: entry icons are now unique within their drawer,
and each card header uses the icon of the entry that opens it.

Two new glyphs, design_thicken and design_rib, are the only ones added —
everywhere else an existing icon already carried the right meaning
(design_revolve, design_offset, design_line, design_point,
design_c_coincident, design_delete).

The Surface drawer BUTTON deliberately keeps design_surface; only its
entries may reuse the solid glyphs, because a menu row carries its own
text label while two adjacent toolbar buttons do not.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

resources/images/design_rib.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#b6b6b6" stroke-width="0.85" stroke-linecap="round" stroke-linejoin="round"><rect x="3" y="18" width="18" height="3" rx="0.5"/><rect x="6" y="6" width="2.5" height="12" rx="0.5"/><path d="M8.5 9L17 18"/></svg>

resources/images/design_thicken.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#b6b6b6" stroke-width="0.85" stroke-linecap="round" stroke-linejoin="round"><path d="M3 10C7 6 11 14 15 10L21 5"/><path d="M3 16C7 12 11 20 15 16L21 11"/><path d="M3 10v6M21 5v6"/></svg>