Copy the page when "+" is pressed on a cloud strategy

[SunkenInTime]

On a local strategy, "+" adds a copy of the page on screen right after it. On a cloud strategy (the web beta and signed-in desktop), it added a blank page. With this PR and #275, cloud "+" copies the page too.

What changes

• The server makes the copy (Copy the page on the server when "+" adds one #275). "+" queues the page add with the page on screen as its source. The server copies that page's items, lineups and image entries onto the new page in the same transaction that adds it. Copies get copy ids (Copy to next or previous page on cloud strategies #267), so items glide when the page turns.
• Edits made just before "+" go ahead of the add in the queue, so the copy has them.
• Once the server accepts the add, the app turns to the new page. "+" follows its own add to the server's answer for up to 5 seconds, even when other work is ahead of it in the queue.
• Images copy whatever state their upload is in. A copied image shows its original's picture (Let an image show another image's picture #276/Find an image's picture by its picture id #277), so an image still uploading is copied too. Its copy shows the picture from this device's bytes at once, and from the cloud once the upload lands.
• Edits to the page that the server refused aren't in its copy, and "+" says so. The copy is made from the server's page. This is checked when "+" is pressed and again once the add lands. The refused edits wait in the sync panel as usual.
• If the add hasn't reached the cloud within 5 seconds (offline, a stalled send, or a backlog), a toast says the page will appear once it does. The add stays queued. The 5 seconds include the send itself.
• A page too large to copy (over 12 MiB) is refused by the server, and the sync panel says so in its own words instead of calling it a conflict.
• Switching strategies mid-"+" stops it before it adds or turns to anything.
• Drawings no longer fade on the turn. A copied stroke counts as its original (TransitionPlanner.drawingsChanged).
• Local "+" is unchanged.

Why the server copies (changed since the first version of this PR)

The first version copied on the client: it added the page with a direct call, then queued a copy of every item. Three rounds of review kept finding races between "the page exists" and "the copies land", the worst being a timed-out add that could still create a blank page after the app said none was added. Having the server copy in one transaction removes that whole class of problem. The field-merge work (#269) set the pattern: an optional op field, so no protocol bump.

Ground truth

Ran the web build in Edge against the dev deployment, which was running this branch's server code, signed in as a test account.

• Page 1 had a lineup, an agent and an image. I pressed "+".
• The app turned to a new Page 2 with the same lineup, agent and image (above).
• Screenshots every 80 ms through the turn show every item staying in place. Nothing faded out and back in.
• On the server, the new page, its image, its lineup and the image's asset row all have the same creation timestamp, so they were written in one transaction. The image's id is a copy id (d8b91641-…~cp1~…), and its asset row points at the original's stored file.

Then, with #276/#277: I placed a new image with its upload held for 20 seconds and pressed "+" during the hold. Frames: placed, then the copy during the hold, then after the upload landed:

The "changes didn't save" toast was only exercised in tests, not in the browser.

Tests

• strategy_page_session_provider_test, group '"+" on a cloud strategy':
  • "+" turns to the server's copy, its images showing their originals' pictures;
  • an answer that comes after the first send still turns to the page;
  • no answer in time says the page hasn't reached the cloud, and leaves the add queued;
  • a send that stalls still answers "+" in time;
  • refused edits to the copied page are named, before the press and in the same send.
• cloud_sync_error_message_test: a page copy too large to make has its own explanation.
• collab_sync_models_test: the field survives storage and withOpId, and an add without it sends no field.
• strategy_op_queue_provider_test: the field survives a restart through the durable outbox.
• page_copy_id_test: a copied stroke counts as its original.
• Full suite on the combined branch: 1902 passed, 9 skipped.

Astra reviewed it three times. Round 1 found six issues, all fixed. Round 2 showed my fixes to three of them were still guessing what the server had done; they're now worked out when "+" is pressed. Round 3 found one more case, an edit refused in the same send as the add, which is now fixed. The "still uploading" warning those rounds were about is gone now that copies share pictures.

Known limit

Two copies of very large pages (over 6 MiB each) sent in the same batch share one budget, so the second is refused. "Keep mine" in the sync panel sends it again. It may need pressing twice, because a refused add is retried at the revision it was first sent with. I left the core queue alone for a case this rare.

Merge order

This PR contains every PR below it and needs the server ones deployed first. Order: #276 and #273 (server), #277, #267 (you look first), #268 and #272, #275, then this. #271 is closed: copies share pictures instead.

ci: test web analyze

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