fix: wire AbortSignal to osascript subprocess for clean cancellation

[ErnestHysa]

Fixes the AbortSignal not wired bug in pick-folder.ts. The osascript subprocess previously only had a 5-min hard timeout with no way to cancel it via AbortSignal. This change adds signal?: AbortSignal support to pickFolder and wires it to execFile for clean cancellation.

[ununununium]

Thanks for the PR — declining this one, with reasoning:

1. The parameter has no caller. The only call site is the POST /api/fs/pick-folder route, and neither it (nor this PR) passes a signal — so the change adds optional API surface with zero behavioral difference. The comment it removes ("we don't wire up an AbortSignal here; the DIALOG_TIMEOUT_MS ceiling protects against a wedged osascript") documents a deliberate decision, and we hold a hard line against speculative parameters in this repo.

2. It targets the pre-rename tree. The app moved from notfair-cmo/ to notfair/ in 0.9.0, so the PR permanently conflicts with main as written.

What would be a welcome resubmission: the complete end-to-end version against notfair/src/server/fs/pick-folder.ts — wire request.signal from the route through pickFolder into execFile, so that when the user abandons the Browse click (tab closed, navigation), the orphaned osascript dialog is actually dismissed instead of lingering up to 5 minutes. That has observable behavior and a clear user benefit. Happy to review it.