feat: CONFIG_NICE_OLED_EXTERNAL_WIDGET — let external modules add widgets to the peripheral screen

[delneet]

What

Adds CONFIG_NICE_OLED_EXTERNAL_WIDGET: an optional hook that lets a separate Zephyr module add a widget to the peripheral screen without forking this repo. When enabled, peripheral screen init calls:

extern int nice_oled_external_widget_init(lv_obj_t *parent);

which the external module provides. That's the whole interface — 18 lines including Kconfig and CHANGELOG.

• Off by default; zero code and zero cost when disabled.
• Enabling it without a providing module fails at link time by design (documented in the Kconfig help).
• Users typically pair it with CONFIG_NICE_OLED_WIDGET_ANIMATION_PERIPHERAL=n / ..._STATIC_IMAGE_PERIPHERAL=n to hand the animation area over.

Why

Addresses the "More animations and fixed images for periphery" TODO without growing this repo: animation content can live in its own modules and evolve independently. First consumer: zmk-peripheral-animations — 10 procedurally rendered effects (plasma, rule 30, Sierpinski zoom, starfield, matrix rain, wireframe cube, …) computed live on the MCU, ~1-2 KB code each instead of stored frames.

Verification

Built against ZMK v0.3.0 (nice_nano_v2, corne_right nice_oled):

• this branch + zmk-peripheral-animations, hook enabled — links and runs the external widget (10 effect variants built)
• hook disabled (default) — byte-for-byte no behavioral change, central and peripheral builds clean