Art frames: real photographic wood (CC0), Mahogany, full-screen fit + proportion fix

- Replace procedural grain with a real fine-grain wood texture (CC0, Poly Haven —
  static/textures/wood-grain.jpg). The three woods share it, recolored per species via
  CSS filter on ::before: Walnut (as-is dark), Oak (lightened/warmed), Mahogany (new —
  deepened/reddened). Miters now apply to all three woods; mat sits between texture and
  miters via z-index.
- Full-screen sizing fixed: image capped (max-height 66vh) so the bottom rail always
  fits — no more cutoff. Rail/mat dialed back to ~1:1 wood:white (was mat-heavy), only
  slightly larger than the page so the proportion matches the small view.

Texture credit (CC0, no attribution required) recorded in static/textures/CREDITS.txt.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/static/textures/CREDITS.txt

@@ -0,0 +1,2 @@
+wood-grain.jpg — "Fine Grained Wood" by Rob Tuytel, Poly Haven (https://polyhaven.com/a/fine_grained_wood)
+License: CC0 (Public Domain). Free to use, no attribution required. Recolored in CSS for frame variants.