Phase F: NetworkMonitor, offline banner, README & architecture docs

- NetworkMonitor: ConnectivityState Flow using ConnectivityManager.NetworkCallback
- ListsScreen & ViewModel: live offline indicator banner when disconnected
- Documentation:
  - docs/ARCHITECTURE.md: system design & tech stack overview
  - docs/SYNC.md: HLC timestamping, op_log outbox & LWW projection specification
  - docs/API.md: REST API endpoint specification
  - README.md: quickstart guide for backend, docker compose & Android app
- Verification: backend & android test suites 100% green ✅

docs/ARCHITECTURE.md

@@ -0,0 +1,51 @@
+# Mitbringsl – Architectural Design Document
+
+## 1. Overview & Paradigm
+
+**Mitbringsl** is a local-first, privacy-focused shopping list application designed as an ad-free alternative to Bring!.
+
+### Local-First Foundation
+- **Source of Truth**: The client’s local SQLite database (via Room) is the authoritative source of truth for user interactions. All mutations (adding items, checking off items, renaming/deleting lists) take immediate effect locally.
+- **Append-Only Op Log**: Operations are logged sequentially to a local outbox queue (`op_log`) before being synced asynchronously to the server.
+- **Offline Resiliency**: The app functions 100% offline. Network connectivity is treated as an opportunistic transport to synchronize operations with the server and other clients.
+
+---
+
+## 2. System Architecture
+
+```
+                  ┌─────────────────────────────────────────┐
+                  │              Caddy (Reverse Proxy)       │
+                  └────────────────────┬────────────────────┘
+                                       │ HTTP / REST
+                  ┌────────────────────▼────────────────────┐
+                  │        Go Backend (net/http + pgx)      │
+                  └─────────┬──────────────────────┬────────┘
+                            │                      │
+                  ┌─────────▼────────┐   ┌─────────▼────────┐
+                  │ PostgreSQL (16)  │   │  OIDC Providers  │
+                  │ (op_log, LWW)    │   │  (Google/Custom) │
+                  └──────────────────┘   └──────────────────┘
+```
+
+---
+
+## 3. Technology Stack
+
+### Backend (Go 1.26)
+- **HTTP Routing**: `net/http` stdlib (Go 1.22+ method & path variables pattern matching).
+- **Database Driver**: `github.com/jackc/pgx/v5` (`pgxpool`).
+- **Database Migrations**: `github.com/golang-migrate/migrate/v4` (embedded via `embed.FS`).
+- **Password Hashing**: Argon2id in PHC format (`golang.org/x/crypto/argon2`).
+- **Authentication**: Opaque session tokens (32 bytes `crypto/rand`, SHA-256 hashed in database).
+- **OIDC Verification**: `github.com/coreos/go-oidc/v3` with JWKS key caching.
+- **Containerization**: Distroless multi-stage Docker build (`gcr.io/distroless/static-debian12:nonroot`).
+
+### Android Client (Kotlin 2.x & Compose)
+- **UI Framework**: Jetpack Compose (Material Design 3).
+- **Architecture**: MVVM with Unidirectional Data Flow (UDF) & `StateFlow`.
+- **Database**: Room (KSP code generation).
+- **Network & Serialization**: Retrofit 2 + OkHttp 4 + `kotlinx.serialization`.
+- **Background Sync**: WorkManager (`HiltWorker` + `CoroutineWorker`).
+- **Dependency Injection**: Hilt.
+- **Clock**: Client-side Hybrid Logical Clock (HLC).