Add local mode startup task and update development documentation

[GustavPersson]

This is the stuff I had to do in order to run locally, and then Claude going absolutely wild on a PR summary

Summary

• Add a LocalUserStartupTask that automatically provisions a default user when running in local development mode
• Overhaul all developer-facing documentation to reflect the current project structure and toolchain

Local Mode Startup Task

The backend now reads the LocalMode configuration flag and conditionally registers a new LocalUserStartupTask. This task runs at application startup when LocalMode is true, ensuring a default local user is available without requiring manual setup through the admin UI.

Changes:

• LocalUserStartupTask.cs (new) — Implements IStartupTask to provision a default local user on startup
• StartupTaskConfiguration.cs — RegisterStartupTasks() now accepts a bool localMode parameter; when true, it registers LocalUserStartupTask alongside the existing InitialUserStartupTask
• Startup.cs — Reads LocalMode from IConfiguration and passes it into RegisterStartupTasks(localMode). Also removes the deprecated EnableEndpointRouting = false option from AddControllers()
• StartupTaskConfigurationTest.cs — Adds a new test (ItShouldRegisterLocalUserStartupTaskInLocalMode) verifying that LocalUserStartupTask is registered when localMode is true, and updates the existing test to explicitly pass localMode: false

Documentation Updates

The existing developer documentation was outdated — it referenced old paths, deprecated CLI commands, and omitted critical setup steps (frontend build, symlink). These changes bring the docs in line with the current monorepo layout and toolchain.

docs/getting-started.md

• Updated framework references from ".NET Core" to ".NET 9" and added "Lustre" as the frontend framework
• Added a Prerequisites section listing all required tools and minimum versions (.NET SDK 9, Node.js 20, Gleam 1.14+, Erlang/OTP 28+, Docker)
• Fixed all file paths to reflect the service/ and tools/ directory structure (e.g. src/Quarter/ → service/src/Quarter/, docker/ → tools/docker/)
• Added new steps for building the frontend (npm ci && gleam build && npm run build) and creating the dist symlink (ln -s ../../dist service/src/Quarter/dist)
• Mentioned npm run watch for development hot-reloading
• Added a Configuration section pointing to appsettings.json and appsettings.Development.json
• Fixed minor copy issues ("start register time" → "start registering time")

README.md

• Replaced the outdated single docker-compose + dotnet quickstart with a complete step-by-step covering PostgreSQL, frontend build, symlink creation, and backend build/run
• Updated docker-compose → docker compose (modern Docker CLI syntax)

docs/authentication.md

• Fixed configuration key references: Application.LocalMode → LocalMode
• Added explicit file path for the config file (service/src/Quarter/appsettings.json)