chore: add CLAUDE.md with project guidance for Claude Code

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+```bash
+# Restore, build, run
+dotnet restore
+dotnet build
+cd Server && dotnet run        # app at http://localhost:5000
+
+# Run all tests
+dotnet test
+
+# Run a single test project
+dotnet test ShiftScheduler.Services.Tests/
+
+# Run a specific test
+dotnet test --filter "FullyQualifiedName~CalendarEventBuilderTests"
+```
+
+Build warnings to ignore (non-breaking, pre-existing):
+- Obsolete QuestPDF `Image` method
+- Possible null reference in `IcsExportService`
+
+## Git Workflow
+
+- All features and fixes are developed on a dedicated branch (never commit directly to `main`)
+- When merging to `main`, squash all branch commits into a single commit with a descriptive message
+
+## Architecture
+
+Blazor WebAssembly hosted on ASP.NET Core. The Server project serves both the API and the WASM client.
+
+```
+Client/      — Blazor WebAssembly frontend (Razor components, pages)
+Server/      — ASP.NET Core host: API controllers, auth, DI wiring (Program.cs)
+Services/    — All business logic: export, transport, calendar sync
+Shared/      — DTOs shared between Client and Server
+ShiftScheduler.Services.Tests/   — xUnit tests for Services layer
+ShiftScheduler.Client.Tests/     — bUnit UI tests for Client components
+Directory.Build.props            — Shared MSBuild properties (TargetFramework, Nullable, ImplicitUsings)
+```
+
+### Key data flow
+
+1. The user selects shifts in the Blazor UI (`Client/Pages/Index.razor`)
+2. Shift data (a `Dictionary<DateOnly, string>`) is POSTed to `ShiftController` in the Server
+3. The controller delegates to services in `Services/` for ICS export, PDF export, transport lookup, or calendar sync
+4. Configuration (shift definitions, transport settings, auth) lives in `Server/appsettings.json`
+
+### Calendar sync providers
+
+`ICalendarService` is the shared interface. Two implementations exist:
+- `GoogleCalendarService` — OAuth token from cookie, Google Calendar API
+- `NextcloudCalendarService` — CalDAV via `Ical.Net`, credentials from `appsettings.json`
+
+`CalendarEventBuilder` contains shared logic for building calendar events and transport descriptions used by both providers.
+
+The API endpoint `POST /api/shift/sync_calendar?provider=google|nextcloud` dispatches to the correct service; unknown providers return 400.
+
+### Authentication
+
+Cookie-based Google OAuth. The `AllowedEmails` authorization policy gates every `ShiftController` endpoint. Authorized emails are configured in `appsettings.json` under `Authentication:AuthorizedEmails`.
+
+### Transport integration
+
+`TransportService` → `TransportApiService` (HTTP) → Swiss public transport API (`transport.opendata.ch`). Results are cached via `IMemoryCache`. `TransportConnectionCalculator` applies safety buffers and selects the optimal connection.
+
+## C# Conventions
+
+- `var` everywhere instead of explicit types
+- Primary constructors
+- File-scoped namespaces, one class per file
+- No code comments
+- `async`/`await` for all I/O; async methods suffixed with `Async`; `CancellationToken` as last parameter
+- `DateOnly` for dates, `TimeOnly` for times, `DateTimeOffset` (Europe/Zurich) for date+time
+- Interfaces on services only when there are multiple implementations or tests require mocking; no `I`-prefix interfaces with a single implementation
+- Use `params IReadOnlyList<T>` (params collections), `Lock` instead of `new object()`, collection expressions and spread operator
+- `IReadOnlySet<T>` / `IReadOnlyDictionary<T>` backed by mutable types for short-lived collections; `FrozenSet<T>` / `FrozenDictionary<T>` / `ImmutableArray<T>` for long-lived cached data
+- No immutable collections otherwise; avoid returning null collections