Migrate web frontend uploads to the GCS resumable scheme

[rtibbles]

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Overview

Migrate the web frontend's upload paths from single-PUT to the opt-in GCS resumable scheme added in #5975, so interrupted uploads resume instead of restarting from byte 0.

Complexity: Medium
Target branch: unstable

Context

Two frontend sites single-PUT the whole file and skip upload when a HEAD on the storage URL returns 200:

• shared/vuex/file/actions.js — uploadFileToStorage
• shared/views/TipTapEditor/.../imageService.js — uploadFileToStorage

Once #5975 lands, upload_url (with resumable: true) returns either a definitive skip or a session URI — the client no longer needs the HEAD check.

The Change

• Pass resumable: true to upload_url; upload to the returned session URI in chunks (256 KiB multiples), resuming from the last persisted offset on interruption.
• Apply to both upload sites.
• On a definitive skip from upload_url, mark the file complete without uploading; remove the client HEAD-existence check.

Out of Scope

• Removing single-PUT / cutover.
• ricecooker client.

Acceptance Criteria

• Both upload sites pass resumable: true and upload to the session URI in 256 KiB-multiple chunks.
• An interrupted upload resumes from the last persisted offset instead of restarting.
• On a definitive skip from upload_url, the file is marked complete without uploading; the client HEAD check is removed.

References

• GCS performing resumable uploads: https://docs.cloud.google.com/storage/docs/performing-resumable-uploads

AI usage

I used Claude (Opus 4.8) to draft this issue from the design established in #5975. I defined the scope and acceptance criteria, and verified the named upload paths against the current code.