Modularize Cahier codebase following Now in Android architecture

Migrate the current single-module monolith into a structured multi-module architecture adhering to Now in Android (NiA) guidelines. By introducing distinct App, Feature, and Core layers driven by centralized Gradle convention plugins, we will isolate complex low-latency inking APIs, improve build performance, and establish clear separation of concerns across the project.

Start by reviewing the current single-module Gradle layout and comparing it with the Now in Android App, Feature, and Core structure. Then inspect how centralized Gradle convention plugins would organize the modules and isolate the inking APIs; done means the monolith is separated into those layers with clear boundaries and improved build performance.

Written by the indexing model from the issue text.