fix(desktop): keep in-flight new chats from vanishing on refresh

Creating several sessions in a row (Ctrl-N, type, send, repeat) and
waiting for one to finish made the other still-running chats disappear
from the sidebar.

Root cause: a new session's first user message isn't flushed to the
SessionDB until its turn is persisted, so the row's message_count stays
0 mid-response. `refreshSessions()` lists with min_messages=1 and then
hard-replaces $sessions. Because every message.complete triggers a
refresh, the moment one session finished, the others (still at
message_count 0) were filtered out of the server page and dropped from
the list.

Fix: merge instead of replace. `mergeWorkingSessions()` preserves any
session that is still in $workingSessionIds but absent from the server
page, so concurrent new chats stay visible until their own turn persists.
Optimistic deletes/archives already remove the row from the previous
list, so a removed session can't be resurrected by the merge.

apps/desktop/src/store/session.test.ts

@@ -2,7 +2,7 @@ import { describe, expect, it } from 'vitest'
 
 import type { SessionInfo } from '@/types/hermes'
 
-import { sessionPinId } from './session'
+import { mergeWorkingSessions, sessionPinId } from './session'
 
 const session = (over: Partial<SessionInfo>): SessionInfo => ({
   archived: false,
@@ -34,3 +34,46 @@ describe('sessionPinId', () => {
     expect(sessionPinId(session({ id: 'tip', _lineage_root_id: 'root' }))).toBe('root')
   })
 })
+
+describe('mergeWorkingSessions', () => {
+  it('returns the server page untouched when nothing is working', () => {
+    const previous = [session({ id: 'a' }), session({ id: 'b' })]
+    const incoming = [session({ id: 'a' })]
+
+    expect(mergeWorkingSessions(previous, incoming, [])).toBe(incoming)
+  })
+
+  it('keeps a still-working session the server omitted', () => {
+    // Repro of the disappearing-sessions bug: A finished and is returned by the
+    // server, but B and C are mid-first-response (message_count 0 in the DB) so
+    // listSessions(min_messages=1) skips them. They must survive the refresh.
+    const previous = [session({ id: 'c' }), session({ id: 'b' }), session({ id: 'a' })]
+    const incoming = [session({ id: 'a', message_count: 2 })]
+
+    const merged = mergeWorkingSessions(previous, incoming, ['b', 'c'])
+
+    expect(merged.map(s => s.id)).toEqual(['c', 'b', 'a'])
+    // The finished session comes from the fresh server payload, not the stale
+    // optimistic copy.
+    expect(merged.find(s => s.id === 'a')?.message_count).toBe(2)
+  })
+
+  it('does not duplicate a working session the server already returned', () => {
+    const previous = [session({ id: 'b' }), session({ id: 'a' })]
+    const incoming = [session({ id: 'b', message_count: 4 }), session({ id: 'a' })]
+
+    const merged = mergeWorkingSessions(previous, incoming, ['b'])
+
+    expect(merged.map(s => s.id)).toEqual(['b', 'a'])
+    expect(merged.find(s => s.id === 'b')?.message_count).toBe(4)
+  })
+
+  it('never resurrects a non-working session the server dropped', () => {
+    // A deleted/archived session is removed from `previous` optimistically and
+    // is not in the working set, so it must stay gone after a refresh.
+    const previous = [session({ id: 'b' }), session({ id: 'gone' })]
+    const incoming = [session({ id: 'b' })]
+
+    expect(mergeWorkingSessions(previous, incoming, ['b']).map(s => s.id)).toEqual(['b'])
+  })
+})