This paper proposes a novel on-device, few-shot learning framework that simultaneously achieves cross-user generalization and individual customization in human activity recognition (HAR) using various sensing modalities. To address the generalization failure of existing HAR models to user-specific variations, we first learn representations that generalize across users, and then directly update a lightweight classifier layer on resource-constrained devices that rapidly adapts to new users with only a small number of labeled samples. We implement and evaluate our framework on a RISC-V GAP9 microcontroller using three benchmark datasets: RecGym, QVAR-Gesture, and Ultrasound-Gesture. Post-deployment adaptation results in accuracy improvements of 3.73%, 17.38%, and 3.70%, respectively. This enables scalable, user-aware, and energy-efficient wearable HAR. The framework has been released as open source.