fix: claim idempotency key, claimant state, bounty OG image, manifest

[privexlabs]

Closes #699
Closes #698
Closes #696
Closes #695

• The contributor who claimed a bounty sees 'This bounty has already been claimed' with no next steps #699: ClaimButton shows a "You're working on this" state with Fixes #N PR instructions when claimedById matches the signed-in user.
• ClaimButton's claim request sends no Idempotency-Key, so the backend rejects every claim with 400 #698: useClaimRace sends an Idempotency-Key header, generated once per claim intent, reused on retry after a failed request and rotated after a definitive outcome.
• Every bounty page shares the generic OG image — no per-bounty share card #696: add issues/[id]/opengraph-image.tsx rendering title, org/repo, reward with asset and status.
• Web app manifest has inverted colours (dark background, near-white theme) and lacks the 192px and maskable icons Chrome requires #695: manifest theme_color now matches the background; add 192px and maskable 512px icons under public/.

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