[RFC] feat(viewer)!: tell openers with events instead of callbacks

[skjnldsv]

Handlers already talk to the viewer with events on their element (loaded, errored, update:*). Openers were the odd one out, with onPrev, onNext, onClose and onEditingChange callbacks in the open options. The viewer app's downloadCallback, which Text used to save unsaved edits before a download, had no equivalent at all. This moves all of it to events, for discussion before 2.0.0 is stable.

open(), openFolder() and compare() resolve with a session, an EventTarget dispatching:

Event	detail	Replaces
update:file	[file]	onPrev, onNext
update:editing	[editing]	onEditingChange
close	[]	onClose
before-download	{ file, waitUntil }	the viewer app's downloadCallback

before-download also goes to the handler's element, which is where Text's editor and its unsaved state live (nextcloud/text#9349). getViewer() dispatches all four for any page, whoever opened the viewer. The names and detail shapes match what handler elements emit.

I kept what the callbacks did across openings. The file shown and editing only reach the latest opening, the way onPrev/onNext came from the current options. close reaches every opening since the viewer opened, the way every onClose was kept. The Files history follows only the latest opening it made: the Files app opens the same file again as the sidebar opens, and unwinding the history once per opening ran history.go() twice. There's a test for that now.

loadMore, enabled, preload and onInit stay functions, since the viewer needs their answer.

This includes #129, which can be closed if this goes in first. It breaks the server's OCA.Viewer shim (apps/viewer/src/legacy.ts), which maps onPrev, onNext and onClose to these options and is what Photos goes through. That needs porting with the bump. Text, Whiteboard and the server's other callers don't pass these options. I dropped the test "tells the others when one of them throws", because dispatchEvent already keeps a throwing listener from stopping the others.

Unit and Playwright tests pass locally.

👾 This pull request was assisted by Claude Code, commits carry an Assisted-by trailer.

[codecov]

Codecov Report

❌ Patch coverage is 97.98658% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.97%. Comparing base (789469f) to head (c446c33).

Files with missing lines	Patch %	Lines
lib/views/Viewer.vue	95.45%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   92.82%   92.97%   +0.15%     
==========================================
  Files          47       49       +2     
  Lines        4069     4172     +103     
  Branches      813      816       +3     
==========================================
+ Hits         3777     3879     +102     
- Misses        273      274       +1     
  Partials       19       19              

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[mejo-]

Code changes look good, and indeed much cleaner interface than before.