perf: optimize session loading and desktop startup (#2545)

* perf: optimize session loading and startup

* fix(chat): stabilize history prepend virtualization

* perf: unblock first session open from startup network contention

Opening the first session after app start waited seconds for its message
fetch. Three independent contributors, each measured via CDP network
capture and Chromium net-log against the packaged desktop app:

- The active-session watchdog fired an uncapped per-directory status poll
  and child-session discovery burst at startup, and other subsystems
  (git checks, global session pages, command/skill discovery) fanned out
  alongside it, saturating the browser's ~6 HTTP/1.1 sockets per origin.
  Add a shared background-network gate (concurrency 3) and route the
  watchdog, poll-shaped git reads (also priority: low), global session
  pages, command/skill loads, and the background update check through it.

- The packaged renderer is cross-origin to the loopback backend, so every
  API call needs a CORS preflight; a few slow OpenCode-proxied requests
  held the whole pool while preflights and interactive traffic queued
  behind them. Lift Chromium's per-host connection cap for loopback via
  ignore-connections-limit in the Electron shell.

- OpenCode initializes each directory lazily on its first request, so the
  first click paid that cost interactively. Warm the last-used directory
  and the three most recently opened projects right after OpenCode
  readiness, sequentially and best-effort, overlapping UI startup.

Validation: new background-network tests, lifecycle warmup test, focused
store/sync tests, UI type-check and lint, dead-code report, node --check
plus electron type-check/lint, and CDP first-open measurements on the
packaged app (message fetch socket queue 5.4s -> 0.03s).

* fix(ui): keep interactive git reads out of background queue

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/sync/sync-context.tsx

@@ -31,6 +31,7 @@ import { bootstrapGlobal, bootstrapDirectory } from "./bootstrap"
 import { retry } from "./retry"
 import { touchStreamingSession, updateChangedStreamingSessions, updateStreamingState } from "./streaming"
 import { countSyncPerformance } from "./performance-diagnostics"
+import { runBackgroundNetworkTask } from "@/lib/background-network"
 import { setActionRefs } from "./session-actions"
 import { setSyncRefs, getAllSyncSessions } from "./sync-refs"
 import { stripSessionDiffSnapshots } from "./sanitize"
@@ -242,6 +243,16 @@ const ACTIVE_SESSION_STATUS_POLL_INTERVAL_MS = 5_000
 const ACTIVE_SESSION_STALE_EVENT_MS = 20_000
 const ACTIVE_SESSION_FULL_RESYNC_COOLDOWN_MS = 15_000
 const CHILD_SESSION_DISCOVERY_INTERVAL_MS = 15_000
+
+// Active-session watchdog network calls run under the shared
+// background-network gate (see lib/background-network.ts). The watchdog walks
+// every initialized child store each tick and fires a status poll plus a
+// child-session discovery list per directory with active candidates — on
+// startup with many cache-hydrated directories that is dozens of simultaneous
+// requests, which would otherwise queue interactive traffic (opening a
+// session) behind them on the browser's ~6 sockets per origin. Later ticks
+// still cover every directory via the per-directory timestamps.
+
 const requestSignature = (items: Array<{ id: string }> | undefined): string => {
   if (!items || items.length === 0) return ""
   return items
@@ -2072,7 +2083,7 @@ export function SyncProvider(props: {
       if (parentSessionIds.length === 0) return
       try {
         const scopedClient = opencodeClient.getScopedSdkClient(directory)
-        const result = await scopedClient.session.list({ directory, limit: 200 })
+        const result: unknown = await runBackgroundNetworkTask(() => scopedClient.session.list({ directory, limit: 200 }))
         const allSessions = ((result as { data?: unknown }).data ?? []) as Session[]
         const state = store.getState()
         const existingIds = new Set(state.session.map((s) => s.id))
@@ -2121,7 +2132,7 @@ export function SyncProvider(props: {
       polling.add(directory)
       try {
         const before = store.getState()
-        const statuses = await resyncDirectorySessionStatuses(directory, store, candidateSessionIds, "monotonic")
+        const statuses = await runBackgroundNetworkTask(() => resyncDirectorySessionStatuses(directory, store, candidateSessionIds, "monotonic"))
         if (!statuses) return
         const needsSnapshot = candidateSessionIds.some((sessionId) => (
           needsSnapshotAfterStatusPoll(before, sessionId, statuses[sessionId])