fix: idempotency key RNG, wallet storage guards, single-request session, returnTo after sign-in

[zane1502]

Summary

• generateIdempotencyKey falls back to Math.random on non-secure origins #706: generateIdempotencyKey falls back to crypto.getRandomValues (v4 UUID) instead of Math.random when crypto.randomUUID is unavailable.
• WalletContext touches localStorage without try/catch in three places — Safari private mode throws #705: WalletContext reads, removes and writes its localStorage key through best-effort try/catch helpers, so storage errors no longer block the mount effect or adoptAndLink.
• AuthContext resolves a session with two sequential requests on every load (/auth/me, then /users/:id) #704: AuthContext.refresh() decodes sub from the JWT and fetches /users/:id directly, keeping /auth/me as the fallback. Also removes the duplicated merge-damage lines in the same function, login type and provider.
• Signing in from a bounty page never returns the user to it #700: ClaimButton sends signed-out users to /connect?returnTo=<path>. A validated same-origin returnTo (new lib/return-to.ts) is kept in sessionStorage across the OAuth round trip and used by CallbackClient after sign-in. CallbackClient is also de-duplicated (it contained two interleaved flows).

Added tests for the UUID fallback and returnTo validation. Builds and tests were not run.
Backend round-tripping of OAuth state is not required by this approach.

Closes #706
Closes #705
Closes #704
Closes #700

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