docs: add project documentation and work log

README.md

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+# ASTRAIN
+
+ASTRAIN is a dark-mode, mobile-first workout logbook. It runs a Blazor WebAssembly frontend and an ASP.NET Core minimal API backend (both on .NET 10) in a single container. The API is served under `/api`, while the UI is served at `/`.
+
+## Features
+- Exercises and routines management
+- Routine runs with last-run weights
+- User identification via 8-character key in the URL
+- SQLite storage via Entity Framework Core
+
+## Local development
+
+### Run API (serves UI when published)
+
+1. Restore/build:
+   - `dotnet restore`
+2. Run the API:
+   - `dotnet run --project src/ASTRAIN.Api/ASTRAIN.Api.csproj`
+
+### Run Client (for UI-only development)
+
+- `dotnet run --project src/ASTRAIN.Client/ASTRAIN.Client.csproj`
+
+## Docker
+
+Build and run the single-container setup:
+
+- `docker compose up --build`
+
+The app will be available at http://localhost:8080
+
+## URLs
+
+- `/exercises` or `/{userId}/exercises`
+- `/routines` or `/{userId}/routines`
+
+Opening a URL without a user id will generate a new user and redirect to `/{userId}/routines`.
+
+## Data
+
+SQLite database file is stored in `src/ASTRAIN.Api/Data/astrain.db` (or `/app/Data/astrain.db` in Docker).