Add display simulator, orientation support, and per-image crop settings

- E-paper display simulator: Python port of the C++ Floyd-Steinberg
  dithering (same palette, same coefficients) with side-by-side preview
  in the web UI. Interactive pan/zoom controls with live re-rendering.

- Frame orientation: landscape / portrait_cw / portrait_ccw setting
  controls logical display dimensions (800x480 vs 480x800). Images are
  rotated to match the physical buffer after processing.

- Display modes: zoom (cover+crop) and letterbox (fit with padding),
  configurable globally and per-image. Zoom mode supports pan_x/pan_y
  (0.0-1.0) to control crop position.

- Settings persistence: frame settings, per-image settings, and frame
  state stored as JSON, surviving restarts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/requirements.txt

@@ -1,2 +1,3 @@
 flask==3.1.*
+numpy>=1.26
 pillow==11.*