feat: add act_quant_mode dropdown to ModelLoader and ModelAutoLoader

Allows selecting activation quantization scale algorithm:
- absmax (default): fast, scale = absmax/7
- mse: search ~5 candidates for min MSE (+1dB quality, ~8% slower)

Passed via config_json at pipeline creation time.

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