feat(scadm): add `scadm export-png` command

[kellervater]

📦 What

Move the export-png.sh script functionality into scadm as a scadm export-png command so both homeracker and homeracker-exclusive can use it without duplicating the script.

💡 Why

cmd/export/export-png.sh is currently duplicated across repos (homeracker core and homeracker-exclusive, with the exclusive version adding -D and --output support). This violates DRY — scadm already manages OpenSCAD installation and knows where the binary lives. 🔧

🔧 Proposed Features

• scadm export-png <input.scad> — basic PNG export
• --output <path> — custom output path
• -D key=value — OpenSCAD variable overrides (repeatable)
• --view <mode> — preset camera angles: iso (default), front, top, back, left, right, bottom, all (renders all views as separate PNGs)
• --imgsize W,H — image dimensions (default: 1920,1080 🖥️)
• --colorscheme <name> — OpenSCAD color scheme (default: BeforeDawn 🌑)
• --camera <params> — raw camera override: translate_x,y,z,rot_x,y,z,dist — overrides --view when both are specified ⚠️

📐 View Presets

View	Camera (translate_x,y,z,rot_x,y,z,dist)
iso	0,0,0,55,0,35,0 (default)
front	0,0,0,0,0,0,0
top	0,0,0,90,0,0,0
back	0,0,0,0,0,180,0
left	0,0,0,0,0,90,0
right	0,0,0,0,0,270,0
bottom	0,0,0,270,0,0,0
all	renders all of the above as <name>_<view>.png

💡 --autocenter --viewall is always applied, so dist=0 lets OpenSCAD auto-fit.

✅ Once Available

Both repos can remove their export-png.sh scripts and use scadm export-png directly. The render-mw-modules.sh wrapper (exclusive repo) can also switch to calling scadm export-png. 🚀