Simplify secure message rendering and pending attachment requests

[ProtonDev-sys]

Optimistic and authenticated encrypted messages duplicated the same card markup, while pending attachment requests used a wrapper object with a dummy failure promise. Share the existing rendering branch and store each pending promise directly, retaining the identity check that prevents stale completions from clearing newer requests.

This removes 14 net lines across two files. The review retained the authenticated attachment manifest, selective downloads, and plaintext memoization because they serve concrete behavior and performance needs.

Validation: 49 attachment-cache, cache-lifecycle, and plaintext-rendering checks passed; optimistic rendering checks, TypeScript, targeted ESLint, desktop standalone build, and git diff --check passed. No live Discord validation was performed.

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