feat: adopt cryptify resume protocol (recovery token + session-expired surface)

[dobby-coder]

Summary

Adopts the cryptify resume/idempotent-retry protocol now exposed by pg-js 1.8.0 (postguard-js#71).

• Bumps @e4a/pg-js to ^1.8.0.
• Wires onUploadInit through pg.email.createEnvelope in the yivi popup and forwards {uuid, recoveryToken} to the background as cryptoPopupUploadInit.
• Persists per-compose-tab in-flight upload records to storage.local (same pattern as PR fix: prevent unencrypted send when encryption is enabled #68's encrypt-state persistence) and clears them on popup done / error and on onAfterSend.
• On background startup, probes each fresh in-flight record with resumeUpload; drops records reported as UploadSessionExpiredError and surfaces a new uploadSessionExpired notification (en + nl).
• Surfaces UploadSessionExpiredError distinctly in the popup error path so a dead Cryptify session no longer shows as a generic "Encryption failed".
• Adds vitest coverage for the new persistence helpers.

The "continue createEnvelope from a rehydrated FileState" piece is intentionally out of scope — pg-js does not yet expose an entry point for it, and the resume probe today is limited to detecting session liveness so the user can be told to resend rather than be left wondering. That follow-up is tracked separately on postguard-js.

Closes #103

Reviewer quickstart

git fetch origin && git checkout dobby/103-pg-js-resume-protocol && npm install && npm run typecheck && npm run build && npm test

Test plan

• npm run typecheck clean
• npm run build succeeds
• npm test — 28 passing, 108 todo, 0 failing (new tests/in-flight-uploads.test.ts covers recordInFlightUpload, persistInFlightUploads, loadInFlightUploads, freshness window, and clearInFlightUpload)
• Manual smoke in Thunderbird: send an encrypted message over a flaky network and confirm a torn chunk PUT recovers via pg-js's internal retry; confirm a fully-expired session (long delay) surfaces the new uploadSessionExpired toast on next interaction instead of "Encryption failed"

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