Add Fit to Frame and Upscale (print tools)

Backend — new /api/print/* router:
- POST /api/print/frame-fit: fit image to 4x6/5x7/8x10/11x14/16x20/20x24/24x36
  and square sizes (4x4/8x8/12x12) at configurable DPI.
  Three modes:
    crop   — center-crop to aspect ratio, Lanczos scale to print res (no AI)
    extend — scale to fill one dimension, AI-inpaint the gap; mirror-fill fallback
    smart  — auto: extend if gap < 15% of frame dimension, else crop
  Auto-detects orientation from image shape; respects explicit portrait/landscape.
- POST /api/print/upscale: Lanczos scale (always) or Real-ESRGAN (if installed)
- GET  /api/print/frame-sizes: frame catalogue with pixel dimensions at 300dpi
- GET  /api/print/upscale/available: reports whether Real-ESRGAN is installed

Frontend:
- modules/image/frame_fit.js: dialog with frame size, orientation, mode, DPI,
  optional extend prompt; shows current image size; result as new layer option
- modules/image/upscale.js: dialog with scale factor (1.5–4×), method selector
  (auto-hides AI option if Real-ESRGAN not available); result as new layer option
- config-menu.js: Fit to Frame... and Upscale... added under Image menu

https://claude.ai/code/session_01B58MaJCU1R6KwBDJCp8AfN

frontend/src/js/config-menu.js

@@ -330,6 +330,16 @@ const menuDefinition = [
 				ellipsis: true,
 				target: 'image/remove_background.remove_background'
 			},
+			{
+				name: 'Fit to Frame...',
+				ellipsis: true,
+				target: 'image/frame_fit.frame_fit'
+			},
+			{
+				name: 'Upscale...',
+				ellipsis: true,
+				target: 'image/upscale.upscale'
+			},
 			{
 				divider: true
 			},