fix: make script help paths lightweight (#1937)

## Summary

Consolidate the script help/startup fixes so standalone helper commands
can show usage without requiring runtime-only dependencies or generating
files.

## Details

Several helper scripts imported optional packages, evaluated newer
annotations, or started generation before users could inspect CLI usage.
On a fresh checkout, this made common help paths fail on missing
packages such as `cairosvg`, `Pillow`, `freetype-py`, `fontTools`,
`pyphen`, `pyserial`, or on Python 3.9 annotation evaluation. A few
generators also treated `--help` as a normal output argument and wrote
files instead of printing usage.

This change keeps runtime dependencies on the code paths that need them,
handles lightweight help/list commands first, and adds explicit `--help`
handling for generator scripts that previously started work.

## Validation

- `python3 <script> --help` across tracked Python scripts, excluding
`scripts/patch_jpegdec.py` because it is a PlatformIO pre-build hook
rather than a standalone CLI
- `python3 scripts/convert_icon.py` still exits with usage for missing
required arguments
- `python3 lib/EpdFont/scripts/fontconvert_sdcard.py --list-presets`
- `python3 -m py_compile` for the changed scripts

scripts/convert_icon.py

@@ -1,18 +1,21 @@
-import sys
 import os
-from PIL import Image
-import cairosvg
 import io
+import sys
 
 threshold = 128
+USAGE = 'Usage: python scripts/convert_icon.py input.png|input.svg output_name width height'
 
 def svg_to_png_bytes(svg_path, width, height):
+    import cairosvg
+
     with open(svg_path, 'rb') as f:
         svg_data = f.read()
     png_bytes = cairosvg.svg2png(bytestring=svg_data, output_width=width, output_height=height)
     return png_bytes
 
 def load_image(path, width, height):
+    from PIL import Image
+
     ext = os.path.splitext(path)[1].lower()
     if ext == '.svg':
         png_bytes = svg_to_png_bytes(path, width, height)
@@ -58,8 +61,11 @@ def image_to_c_array(img, array_name):
     return c
 
 def main():
-    if len(sys.argv) < 5:
-        print('Usage: python convert_image.py input.png output_name width height')
+    if any(arg in ('-h', '--help') for arg in sys.argv[1:]):
+        print(USAGE)
+        sys.exit(0)
+    if len(sys.argv) != 5:
+        print(USAGE)
         sys.exit(1)
     input_path, output_name, width, height = sys.argv[1:5]
     array_name = output_name.capitalize() + 'Icon'
@@ -77,4 +83,4 @@ def main():
     print(f'Wrote {output_path}')
 
 if __name__ == '__main__':
-    main()
+    main()