Add CLAUDE.md with build commands and architecture overview

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.
+
+## Build & development commands
+
+```bash
+./gradlew assembleDebug          # debug build
+./gradlew assembleRelease        # release build (requires signing keystore)
+./gradlew ktlintCheck            # lint check (also runs on every git commit via pre-commit hook)
+./gradlew ktlintFormat           # auto-fix lint issues
+./gradlew test --no-daemon       # unit tests (also runs on every git push via pre-push hook)
+./build.sh                       # builds both offline and free release variants to ~/release/
+```
+
+Git hooks are auto-installed via the root `build.gradle` preBuild task — no manual setup needed.
+
+## Build flavors
+
+| Flavor | Purpose |
+|--------|---------|
+| `free` (default) | Online variant with full networking (OkHttp, Coil, relay connectivity) |
+| `offline` | No network stack; use `BuildFlavorChecker.isOfflineFlavor()` to guard network code |
+
+## Architecture
+
+### Request ingestion — three paths
+
+External apps and relays reach the signer through three distinct paths:
+
+1. **`nostrsigner://` Intent** → `SignerActivity` → parsed by `IntentUtils` → shown as bottom sheet
+2. **ContentProvider IPC** → `SignerProvider` → synchronous signing via `runBlocking`
+3. **NIP-46 relay events** (kind 24133) → `NotificationSubscription` → `EventNotificationConsumer` → `BunkerRequestUtils`
+
+All three paths converge on `Account.sign()` / encrypt/decrypt methods backed by `NostrSignerInternal`.
+
+### Global state — `Amber.kt`
+
+`Amber` is the Application class and acts as the DI container. Key singletons it owns:
+
+- `applicationIOScope` — `CoroutineScope(Dispatchers.IO + SupervisorJob() + exceptionHandler)`, used for all background work
+- `client: NostrClient` — the Quartz Nostr relay client
+- `notificationSubscription` — keeps the NIP-46 filter alive in the background
+- `profileSubscription` — active only in the foreground (paused in background to save battery)
+- `isStartingAppState: MutableStateFlow<Boolean>` — set to `true` during `runMigrations()`; code that must wait for startup calls `isStartingAppState.first { !it }`
+- `settings.killSwitch` — when true, all relays are disconnected; checked before every relay operation
+
+### Per-account isolation
+
+Every `npub` gets its own:
+- `SharedPreferences` file (`prefs_${npub}`)
+- `AppDatabase` (`amber_db_${npub}`) — apps + permissions
+- `LogDatabase` — operation logs
+- `HistoryDatabase` — request history
+
+All databases are lazy-loaded and cached in `ConcurrentHashMap`s in `Amber`. Account data (including decrypted keys) is loaded via `LocalPreferences.loadFromEncryptedStorage()` and cached in `LargeCache`.
+
+### Permission system
+
+Permissions are stored in `ApplicationEntity` + `ApplicationPermissionsEntity` (Room). Each permission entry has:
+- `rememberType` — auto-accept, auto-reject, or always-ask
+- `acceptUntil` / `rejectUntil` — time-bound grants
+- `kind` — event-kind-specific rules
+
+Before showing the approval UI, all three ingestion paths query the database; if a matching auto-accept rule exists, signing proceeds silently.
+
+### Key files
+
+| File | Purpose |
+|------|---------|
+| `Amber.kt` | Application singleton, Nostr client, relay connectivity |
+| `LocalPreferences.kt` | Account/settings persistence, encrypted key storage |
+| `IntentUtils.kt` | Parses `nostrsigner://` URIs, creates `IntentData` objects |
+| `SignerProvider.kt` | ContentProvider IPC signing |
+| `BunkerRequestUtils.kt` | NIP-46 protocol handling, relay responses |
+| `AccountStateViewModel.kt` | Auth state, account switching, toast notifications |
+| `ConnectivityService.kt` | Foreground service, network monitoring, relay reconnection |