docs: add FEATURES.md — implemented feature documentation

[Copilot]

No documentation existed for the implemented features beyond a high-level business requirements file. FEATURES.md fills that gap with a developer-oriented reference.

What's documented

• Feature status table — quick scan of what's live, in-progress, or out of scope
• Task model — Task and CreateTaskBody TypeScript interfaces with field semantics
• REST API reference — GET /tasks and POST /tasks with request/response shapes and validation error messages
• Frontend layer — component hierarchy, rendering conditions for loading/empty/error states, refresh behaviour
• Data & services — useTasks hook contract, taskApiService / taskService responsibilities, seed data composition (108 tasks, ~25% completed, cycling priorities)
• Architecture diagram — browser → Vite proxy → Express flow with design principles
• CSS class reference — BEM class names mapped to visual treatment
• Run instructions — dev, individual server, and production build commands

Original prompt

create a document for the implemented feature for this application

Created from Copilot CLI via the copilot delegate command.