fix(ui): expire steward tokens with malformed exp

[lalalune]

Summary

• Treat Steward JWTs with non-numeric or non-finite exp claims as expired
• Add regression coverage so malformed truthy exp values cannot survive the 401 keep-path

Evidence

• bun run --cwd packages/ui test -- src/cloud/shell/StewardProviderShared.test.ts src/cloud/shell/StewardProviderRuntime.test.tsx src/cloud/public-pages/pages/auth/email-callback-page.test.tsx
• bunx biome check packages/ui/src/cloud/shell/StewardProviderShared.ts packages/ui/src/cloud/shell/StewardProviderShared.test.ts
• bun run --cwd packages/ui typecheck
• bun run --cwd packages/app audit:app passed 349 captures on the reviewed patch: broken=0, needs-work=0

Follow-up from review of #11067.

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[lalalune]

QA review: strict-numeric exp check is correct per JWT NumericDate (a string exp is malformed and must not survive the 401 keep-path). Ran StewardProviderShared.test.ts on the PR head in an isolated worktree: 8/8 pass including the new regression test.

[claude]

Claude encountered an error —— View job

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