Add design canvas: two TE-inspired directions before the React port

Design inspiration is the Teenage Engineering EP-133 manual: flat fills,
heavy outlines, hard zero-blur offset shadows, one orange accent,
uppercase technical type, monospaced numerals.

Six artboards drawn at the real measured 360x735 with the actual
vertical budget (44px pickers, 485px panel, 116px info, 58px tabs), so
a mockup cannot fit in a way the device won't:

- Current       the app as it stands, for comparison
- A Manual      light ground, 2px black outlines, black pads
- B Blueprint   pure black, 1px white hairlines, no fills
- Specimen      type candidates and palette, incl. which orange

Type is identical across both directions so the light/dark choice is
not confounded with the font choice. Green/red drill feedback is
replaced by orange fill vs dashed outline - neither exists in this
palette.

The seeded canvas payload is gitignored; these files are its source.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -18,3 +18,6 @@ android/.idea/
 *.aab
 *.keystore
 !debug.keystore
+
+# Seeded design canvas — regenerated from design/canvas/*.dc.html
+design/canvas/sp404-layers-redesign.html