Make Gemma 26B default and align Web UI OCR path

[amariichi]

Summary

• make Gemma 4 26B-A4B the default model and document mmproj coexistence with E4B
• apply Gemma reasoning suppression to all Gemma 4 models, not only the old default
• move the Web UI onto the same grounding/OCR path as the shortcut client and harden fallback extraction

Verification

• bash -n start.sh
• python3 -m compileall app