M1.4 Tier-aware app scaffold (PWA shell + worker harness + storage interface) #10

Part of #1 (M1).

Goal

The minimal app skeleton, designed tier-aware from day one: a PWA shell, a Web Worker harness, and a storage interface abstract enough to back Tier 0/1/2 without a rewrite. No business logic yet.

Acceptance

• Installable PWA that loads offline after first visit.
• A worker that does real work off the main thread.
• A storage interface with an OPFS implementation and a clean tier boundary (Tier 0 must not be thrown away to reach Tier 1).