revert: restore main to #3941 deployed-good baseline (incident recovery) (#3947)

Production was rolled back (Vercel deployment promotion) to the #3941 build
after #3942 + #3113 broke the dashboard: panels out of order, most panels
stuck on "Loading...", Live News never loaded, panel layout not persisting,
and repeated /api/user-prefs 409s. The deployment rollback fixed the live
app, but git main still pointed at #3945 — so any merge/redeploy of main
would silently reintroduce the entire breakage.

This commit makes main's tree identical to the #3941 build that is currently
serving production, undoing the net effect of:
  - #3939 (ML worker → Hugging Face Transformers)
  - #3942 (cloud panel-order sync)
  - #3113 (barrel elimination + variant panel chunks + SW precache change)
  - #3943 (single panels chunk follow-up)
  - #3945 (panel preference sync stabilization)

Nothing is permanently discarded — this is a revert. The genuinely valuable
pieces will be re-landed as separate, isolated, individually-verifiable PRs:
  - cloud-prefs upload coalescing (#3945) + dead-key fix (#3942), WITHOUT
    syncing panel-order (panel order stays local-only)
  - barrel-file elimination (#3113), measured, on its own
  - ML worker migration (#3939), on its own
The variant chunk split + SW precache change stay parked until panel modules'
top-level service clients are lazy-initialized.