feat: cross-browser/cross-device compatibility sweep
Browser compat gate (WASM/ESM check + structuredClone polyfill),
structured WebAuthn error handling with user-facing messages,
email-only login mode when passkeys unavailable, Firefox passphrase
fallback for document encryption (salt storage, modal UI, key
derivation bridge), CSS flex gap fallbacks for Safari <14.1,
MapLibre CDN load error handling, and server-side auth error
improvements with proper HTTP status codes.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>