Improve cold start performance: cache session list for instant display

[lucharo]

Problem

Every time the iOS app is cold-started (process killed by OS while backgrounded), users see a loading spinner for 1-3 seconds before sessions appear. This happens on every reopen because:

1. Zustand store initializes with isDataReady: false and empty sessions
2. SessionsListWrapper shows a spinner when useSessionListViewData() returns null
3. The app must fetch /v2/sessions, decrypt all session encryption keys (sequential loop), decrypt metadata + agentState per session, then call applyReady()

Sessions are not persisted to MMKV — only drafts, permission modes, and settings are cached locally.

Root Cause Analysis

In sessionSnapshot.ts, both decryption loops are fully sequential (await inside for):

• Lines 75-90: Sequential encryption key decryption (decryptEncryptionKey per session)
• Lines 95-149: Sequential session metadata/agentState decryption

In sync.ts:

• applyReady() (the only place isDataReady becomes true) is called in exactly one place: #init() line 527, after sessionsSync.awaitQueue() + machinesSync.awaitQueue() complete
• isDataReady starts as false and is never persisted

Proposed Fix

Phase 1: Parallelize decryption (low risk, high impact)

• Use Promise.all (with concurrency limit) for the key decryption loop
• Use Promise.all for the session metadata/agentState decryption loop

Phase 2: Cache session list to MMKV (medium risk, highest impact)

• After applySessions(), serialize sessionListViewData to MMKV
• On cold start, load cached data immediately before fetch completes (stale-while-revalidate)
• Set isDataReady: true immediately if cache exists

Environment

• iOS (Happier mobile app)
• Affects all cold starts (any time iOS kills the app process in background)
• More noticeable with many sessions (50+)