[codex] improve README architecture overview

[alvin000009238]

Summary

• Replaces the placeholder README architecture Mermaid diagram with a fuller repo and Android runtime overview.
• Adds the major UI, ViewModel, data, background worker, widget, notification, analytics, test, and external-service boundaries.
• Adds a short note describing the core runtime data flow from Compose UI through Repository and SchoolGradeClient to the school system.

Impact

This makes the README architecture section useful for new contributors reviewing the project structure and runtime boundaries.

Validation

• Documentation-only change; Android tests were not run.
• Inspected the README diff to confirm only README.md changed.

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[gemini-code-assist]

Code Review

This pull request updates the README.md file to include a comprehensive Mermaid flowchart illustrating the architecture, subgraphs, and data flow of the Android application, along with a brief textual description of the core data flow. Feedback was provided to improve the completeness of the diagram by connecting key UI screens (such as Grades, ScheduleUi, and SettingsUi) to their respective ViewModels to accurately represent the data flow between the UI and presentation layers.

Important

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[gemini-code-assist]

In the Mermaid diagram, several key UI screens defined in the UI subgraph (such as Grades, ScheduleUi, and SettingsUi) are defined but not connected to their respective ViewModels (ScoreVm, ScheduleVm, SettingsVm). Adding these connections will make the architecture diagram complete and accurately represent the data flow between the UI and presentation layers.

Suggested change

	Login --> ScoreVm
	ScoreVm --> GradeRepo
	Login --> ScoreVm
	Grades --> ScoreVm
	ScheduleUi --> ScheduleVm
	SettingsUi --> SettingsVm
	ScoreVm --> GradeRepo