ZDM v0.3.0

Rework download scheduling to reconcile top-N active downloads

- Scheduler now demotes running downloads pushed out of the active
  window (e.g. by drag-and-drop reorder) instead of only ever filling
  empty slots, and promotes whatever should run in their place.
- Queue start/stop is now atomic: stopping holds every member
  (previously only the actively-downloading one), starting requeues
  held downloads and lets the scheduler respect the concurrency cap
  instead of force-starting everything. Fixed the same bug in the
  global pause/resume-all control.
- resume_download/retry_download now fall back to disk-resume-or-fresh-
  start when there's no live engine task, needed now that a download
  can be Paused without ever having started.
- Add Download now probes URLs (single and batch, bounded concurrency)
  before submitting, showing total size and letting broken links be
  dropped via a Download Anyway / Cancel prompt instead of silently
  failing after the fact.
- Added a 5-color accent picker to Settings.
- Bump version to 0.3.0; fold in the still-uncommitted Gatekeeper
  README fix and untrack the stray .claude session-lock file.