perf: Make NIP-17 inbox sync on-demand for optimal login performance

This commit eliminates automatic inbox initialization on login, preventing
unwanted network requests and heavy I/O operations. Inbox sync now only
activates when users explicitly enable it.

## Problem

When logging in, the app would automatically:
- Initialize gift wrap service immediately
- Auto-enable inbox sync without user consent
- Load encrypted content from Dexie
- Wait up to 1 second for cache readiness
- Fetch inbox relay lists from network
- Subscribe to gift wrap events
- Open persistent relay connections

This caused:
- App hangs during login (network/IO blocking)
- Unwanted network activity before user opts in
- Poor performance on initial load
- Unnecessary resource consumption when DMs not needed

## Solution

### 1. On-Demand Initialization (useAccountSync.ts)
**Before**: Auto-init and auto-enable on every login
**After**: Watch settings$ and only init when user enables

```typescript
// Only initialize when user explicitly enables inbox sync
const settingsSub = giftWrapService.settings$.subscribe((settings) => {
  if (settings.enabled && giftWrapService.userPubkey !== pubkey) {
    giftWrapService.init(pubkey, signer);
  }
});
```

### 2. Early Exit for Disabled State (gift-wrap.ts)
**Before**: Always loaded cache and relays, then checked enabled flag
**After**: Check enabled FIRST, exit early if disabled

```typescript
async init(pubkey: string, signer: ISigner | null) {
  // Set basic properties
  this.userPubkey = pubkey;
  this.signer = signer;

  // Early exit if disabled (prevents expensive operations)
  if (!this.settings$.value.enabled) {
    return;
  }

  // Only do expensive operations when enabled
  await getStoredEncryptedContentIds();
  await this.waitForCacheReady();
  this.loadInboxRelays();
  // ...
}
```

### 3. Updated Documentation
- Clarified on-demand initialization flow
- Updated lifecycle documentation with performance notes
- Changed "auto-enable" section to "on-demand" section

## Performance Impact

**Login Performance**:
- ✅ No automatic Dexie reads
- ✅ No cache waiting (up to 1s saved)
- ✅ No network requests for inbox relays
- ✅ No relay subscriptions until needed
- ✅ Instant login when DMs not needed

**User Control**:
- Users explicitly opt-in via InboxViewer toggle
- Clear UI feedback about enabling inbox sync
- No surprise network activity

**When Enabled**:
- Full functionality identical to before
- All optimizations from previous commits preserved

## Testing

- ✅ All 864 tests pass
- ✅ Build succeeds with no errors
- ✅ Verified on-demand initialization flow
- ✅ Confirmed no auto-init on login

## Files Changed

- `src/hooks/useAccountSync.ts` - Watch settings, init only when enabled
- `src/services/gift-wrap.ts` - Early exit if disabled, expose userPubkey
- `src/components/InboxViewer.tsx` - Updated comments
- `docs/gift-wrap-architecture.md` - Updated flow and lifecycle docs

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/gift-wrap-architecture.md

@@ -124,14 +124,15 @@ Chat Component
 
 ### Receiving Messages (Inbox Flow)
 
-1. **Account Login** → `useAccountSync` calls `giftWrapService.init(pubkey, signer)`
-2. **Fetch Inbox Relays** → Load kind 10050 from user's outbox relays
-3. **Subscribe to Gift Wraps** → Open subscription to inbox relays for `kind 1059` with `#p` = user pubkey
-4. **Gift Wrap Arrival** → EventStore receives event → GiftWrapService detects new gift wrap
-5. **Decrypt** (if auto-decrypt enabled) → Call `unlockGiftWrap(event, signer)`
-6. **Extract Rumor** → Get kind 14 DM from gift wrap inner content
-7. **Group into Conversations** → Compute conversation ID from participants → Update `conversations$` observable
-8. **UI Update** → InboxViewer/ChatViewer re-renders with new messages
+1. **User Enables Inbox Sync** → User toggles "Enable Inbox Sync" in InboxViewer settings
+2. **Service Initialization** → `useAccountSync` detects enabled setting and calls `giftWrapService.init(pubkey, signer)`
+3. **Fetch Inbox Relays** → Load kind 10050 from user's outbox relays
+4. **Subscribe to Gift Wraps** → Open subscription to inbox relays for `kind 1059` with `#p` = user pubkey
+5. **Gift Wrap Arrival** → EventStore receives event → GiftWrapService detects new gift wrap
+6. **Decrypt** (if auto-decrypt enabled) → Call `unlockGiftWrap(event, signer)`
+7. **Extract Rumor** → Get kind 14 DM from gift wrap inner content
+8. **Group into Conversations** → Compute conversation ID from participants → Update `conversations$` observable
+9. **UI Update** → InboxViewer/ChatViewer re-renders with new messages
 
 ### Sending Messages (Outbox Flow)
 
@@ -161,18 +162,19 @@ Chat Component
 
 ### Lifecycle
 
-**Init** (on account login):
+**Init** (when user enables inbox sync):
 ```typescript
 giftWrapService.init(pubkey, signer)
-  1. Load persisted encrypted content IDs from Dexie
-  2. Wait for cache readiness (prevents race condition)
-  3. Subscribe to user's kind 10050 (inbox relays)
-  4. Load stored gift wraps from Dexie into EventStore
-  5. Subscribe to EventStore timeline for real-time updates
-  6. Open persistent relay subscription for new gift wraps
+  1. Check if enabled (early return if disabled for performance)
+  2. Load persisted encrypted content IDs from Dexie
+  3. Wait for cache readiness (prevents race condition)
+  4. Subscribe to user's kind 10050 (inbox relays)
+  5. Load stored gift wraps from Dexie into EventStore
+  6. Subscribe to EventStore timeline for real-time updates
+  7. Open persistent relay subscription for new gift wraps
 ```
 
-**Cleanup** (on account logout):
+**Cleanup** (on account logout or disable):
 ```typescript
 giftWrapService.cleanup()
   1. Unsubscribe from all observables
@@ -180,6 +182,8 @@ giftWrapService.cleanup()
   3. Clear in-memory state
 ```
 
+**Performance Note**: Init is only called when user explicitly enables inbox sync via InboxViewer toggle. This prevents automatic network requests and heavy I/O operations on login.
+
 ## Cache Strategy
 
 ### Encrypted Content Persistence
@@ -231,13 +235,16 @@ giftWrapService.cleanup()
 - Clear UI feedback about why sending is blocked
 - Relay lists can be fetched in background without blocking UI
 
-### Auto-Enable Inbox Sync
+### On-Demand Inbox Sync
 
-**Default**: Inbox sync is **auto-enabled** on first login to ensure users receive DMs.
+**Default**: Inbox sync is **disabled** on login for optimal performance.
 
-**Rationale**: Better UX to auto-enable with opt-out than require manual setup.
+**Rationale**: Prevents automatic network requests and heavy I/O operations on login. Users must explicitly enable inbox sync to receive DMs.
 
-**User Control**: Settings UI allows disabling inbox sync and auto-decrypt.
+**User Control**:
+- Enable/disable inbox sync via toggle in InboxViewer
+- Configure auto-decrypt behavior in settings
+- Service initializes only when explicitly enabled
 
 ### Relay List Privacy