[codex] Cover clipboard fallback failure paths

[Dreamstore2046]

Summary

• add SettingsTab coverage for clipboard failure showing the fallback toast and selectable text
• add a source audit test that keeps direct navigator.clipboard / document.execCommand usage confined to dashboard/src/lib/clipboard.ts
• lock in the existing shared copyText fallback path across Wallet and Settings copy buttons

Closes #218

Validation

• npm test -- dashboard/src/__tests__/clipboard.test.ts dashboard/src/__tests__/settings-tab.test.tsx dashboard/src/__tests__/wallet-tab.test.tsx dashboard/src/__tests__/clipboard-audit.test.ts
• git diff --cached --check

Notes

• The shared copyText helper already performs the navigator.clipboard.writeText try/catch plus document.execCommand('copy') fallback; this PR adds the missing component failure coverage and a regression audit so future clipboard calls do not bypass it.

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