feat(desktop): split cron sessions into their own sidebar section

Scheduler sessions (source=cron) were listed in recents, where their
`[IMPORTANT: …]` first-message previews spammed the list — and because
cron runs are always newest, a burst of them consumed the whole recents
page budget and starved real conversations (sidebar showed 0 sessions).

Recents and cron jobs are now two independent lists:
- Backend: /api/sessions + /api/profiles/sessions accept source /
  exclude_sources; session_count gains exclude_sources. Recents query
  excludes cron; the cron section queries source=cron.
- Desktop: separate $cronSessions store + refreshCronSessions fetch, a
  collapsed (persisted) "Cron jobs" section below Sessions that only
  renders when cron sessions exist, with its own bounded scroller.

apps/desktop/src/store/session.ts

@@ -76,6 +76,10 @@ export const $connection = atom<HermesConnection | null>(null)
 export const $gatewayState = atom('idle')
 export const $sessions = atom<SessionInfo[]>([])
 export const $sessionsTotal = atom<number>(0)
+// Cron-job sessions (source === 'cron') are fetched as their own list so the
+// scheduler's always-newest sessions never crowd recents out of the page
+// budget. Powers the collapsed "Cron jobs" sidebar section.
+export const $cronSessions = atom<SessionInfo[]>([])
 // Listable conversation count per profile (children excluded), keyed by profile
 // name. Lets the sidebar scope its "Load more" footer to the active profile so a
 // huge default profile doesn't keep "Load more" visible while browsing a small
@@ -119,6 +123,7 @@ export const setConnection = (next: Updater<HermesConnection | null>) => updateA
 export const setGatewayState = (next: Updater<string>) => updateAtom($gatewayState, next)
 export const setSessions = (next: Updater<SessionInfo[]>) => updateAtom($sessions, next)
 export const setSessionsTotal = (next: Updater<number>) => updateAtom($sessionsTotal, next)
+export const setCronSessions = (next: Updater<SessionInfo[]>) => updateAtom($cronSessions, next)
 export const setSessionProfileTotals = (next: Updater<Record<string, number>>) =>
   updateAtom($sessionProfileTotals, next)
 export const setSessionsLoading = (next: Updater<boolean>) => updateAtom($sessionsLoading, next)