feat(swa): add proof-of-work mining and feed difficulty filter

[Ryback2501]

Summary

• Implement NIP-13 proof of work on both the produce and consume sides: outgoing notes can mine a nonce tag to a chosen target difficulty, and the feed shows and can filter by committed difficulty.

Changes

Proof of work core (swa/)

• proofOfWork.js — new module: countLeadingZeroBits (leading-zero-bit difficulty of an id), getEventDifficulty (difficulty of an event), and mineProofOfWork (increments a nonce tag until the id meets a target difficulty). Pure functions, reusing serializeEvent/getEventId.
• proofOfWork.test.js — unit tests incl. the NIP-13 worked example (36 bits), base-tag preservation, and that mined ids meet the target.

Signing (swa/, browser/)

• nostr.js (both copies) — createEvent now accepts an optional createdAt, so a mined id is reproducible through signing instead of being regenerated.
• signer.js — LocalSigner.signEvent forwards draft.created_at, making it consistent with the extension path (which already honored it).
• Tests added for the reproducible-createdAt behavior in nostr.test.js and signer.test.js.

UI wiring (swa/)

• app.js — createOwnEvent mines the nonce before signing when difficulty > 0 (works for both signers); post/reply/quote pass the configured difficulty; a "min PoW" feed filter toggles card visibility (preserving the 1:1 card/event render invariant).
• store.js — postDifficulty preference persisted in localStorage (default 0 = off).
• feedView.js — renders a ⛏ PoW badge and stamps data-pow on each card.
• index.html / style.css — proof-of-work selector in the compose panel and a min-difficulty selector in the feed filters.

Test plan

• Set "Proof of work" to 8 bits, post a note; confirm "Mining proof of work…" then the note publishes and shows a ⛏ 8+ badge.
• Reply and quote with PoW enabled; confirm both carry a nonce and a badge.
• With PoW off, posting works unchanged and no badge shows.
• Set the feed "min PoW" filter to ≥ 8 bits; confirm low-PoW notes are hidden and clearing the filter restores them.
• Reload the page; confirm the compose difficulty selection persists.
• Verify with a signing extension (NIP-07): a mined note still signs and verifies.