feat(decrypt): smooth transitions between Yivi, decrypt, and done states

[rubenhensen]

Summary

• Hold briefly on the Yivi success animation before flipping to the decrypt progress banner
• Fade between Ready / Decrypting / Done states and slide the progress bar in/out
• Keep the success banner through Done with a distinct completion message (doneMessageComplete)

Test plan

• Decrypt a file end-to-end: Yivi success → progress banner → done banner transitions feel smooth
• Banner message switches from "being downloaded" to "have been downloaded" on completion
• NL locale renders the new message correctly
• Retry status still appears during decrypt when applicable

[dobby-coder]

Dobby has received the request! Routing to the right specialist now...

[dobby-coder]

Code review

UI animation polish for the decrypt page: 1400ms hold on Yivi success, fade/slide transitions between Ready/Decrypting/Done, distinct Done-state message. Build, svelte-check, and lint pass; no functional regressions.

Rule compliance

One soft a11y note: the repo has no prefers-reduced-motion handling and this PR adds several transitions. Strictly WCAG AAA territory, not AA — flagging only because the rule explicitly calls out animations.

[dobby-coder]

[Code review] pendingDecryptFlip is never cleared on unmount — if the user navigates away during the 1400ms hold, the timer fires on a torn-down component. Add an onDestroy that calls clearTimeout.

[dobby-coder]

[Code review] .success-banner :global(.container .label) { display: none } is broad enough to leak if another .container ever lands inside the banner. Scope it to the DecryptionProgress component or use a more specific selector.