perf(app): spawn concurrency cap, background prefetch, Today default

[iamtoruk]

App half of the desktop-slowness fix (root-caused on a data-heavy machine: each CLI spawn costs 1.5-4s there, and the app multiplied that by spawning without limit).

• Spawn concurrency cap (2) with two priority classes in app/electron/cli.ts: interactive (user-visible polls, Settings mutations) always dequeues before background; cache/coalesce hits never take a slot; per-call timeout starts at real spawn, not while queued; killAll also settles queued waiters so quit can't hang.
• Prefetch loop tamed: provider warms are background-priority (add-only IPC arg, older preload degrades gracefully), stagger raised 400ms → 2000ms so the loop is genuinely paced, and warms hold while a user fetch is in flight instead of racing it. No whole-set re-warm on period change (session-lifetime warmedKeys guard kept).
• Default period is now Today (was Last 30 Days) when no Settings preference is saved — a saved preference still wins. Also the cheapest period to compute.

Tests: app suite 409 passed (6 new: scheduler cap/preemption/coalesce/cancel, IPC priority plumbing, default-period); typecheck + electron build clean.