fix(app): keep live thinking state across a full session refetch

[charliezong18]

What

fetchSessions() hardcodes thinking: false on every session it decodes, and applySessions() overwrites rather than merges that field. The server's session record has no thinking state at all — it only ever arrives through activity ephemerals — so any full refetch declares every running session idle until the next heartbeat.

Why it's visible

Creating a session triggers exactly that refetch:

new-session update → sessionsSync.invalidate() (sync.ts:2276) → fetchSessions() → thinking: false (sync.ts:1006) → applySessions() (storage.ts:457, the spread overwrites it)

Two symptoms follow, and they hit every running session, not just the one being created:

1. The pulsing dot in the session list freezes for a beat, until the next activity ephemeral restores thinking.
2. The unread detector in applySessions (storage.ts:600-613) reads the same thing as wasActive && isNowIdle and marks the session unread — a session that never stopped working shows the solid unread dot until you open it.

Fix

Preserve the thinking state already held instead of resetting it. Genuine thinking: false transitions still arrive through the activity accumulator and turn-end message events, which pass an explicit value; this only stops the refetch path from inventing one.

Notes

Complements #1567 (unread markers dropped by rebuilds that omit unreadSessionIds) and #1577 (unread masking a live session). Those fix how the marker is rendered and retained — this fixes a source of markers that should never have been set. No file overlap with either.

Testing

pnpm typecheck and the 793-test suite pass. Verified by hand on a fork build: with several sessions running, creating a new session no longer freezes their pulsing dots or flips them to unread.

No unit test — the zustand store can't be imported under vitest (react-native flow types break the transform), which is why no existing test loads it.

No screenshot: the symptom is a one-to-two second freeze of a pulsing animation, which a still frame cannot show.

[charliezong18]

Revised after a self-review pass, two changes:

• Preservation is now gated on the server-reported active: a dead session can never send the clearing ephemeral, so its remembered thinking: true would otherwise be immortal — masked while disconnected, but resurfacing on resume and re-triggering the exact spurious-unread this PR fixes.
• The preserved values are resolved synchronously with the apply instead of inside the decrypt loop, closing a window where an activity ephemeral landing mid-fetch (agent just finished) could be overwritten by a stale pre-await snapshot.

Also worth noting for review: this lives in fetchSessions rather than storage.applySessions deliberately — the storage-level apply is shared with flushActivityUpdates, whose incoming thinking is authoritative and must overwrite; the fetch path is the only safe synthesis point.