feat(viewer): name the element that shows a file, to render it elsewhere

[skjnldsv]

The preview of a file link (the Files reference widget, shown in Talk, Text, Collectives and the Smart Picker) used to render the file inline: the video playing, the PDF scrolling, right in the message. It found the handler's Vue component through OCA.Viewer.availableHandlers, which is gone, so the widget always falls back to its static card.

getViewer().elementFor(file) gives the tag name of the handler's custom element, once it's defined, or nothing when no handler takes the file or it can't be read. It reads the shared registry and loads nothing of the viewer itself. The new embedded prop on ViewerProps tells the handler it is shown inline: videos and sounds then wait to be played instead of starting on their own. It's a hint not to take over the page: editing is usually better left to the viewer, which the caller can open on the file.

The server widget gets ported to it once this is released, and keeps its own card as the fallback.

Unit tests cover the element being named without loading the viewer, onInit running first, nothing for a file no handler takes or that can't be read, and autoplay off when embedded. The README has a "Show a file outside the viewer" section.

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

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.39%. Comparing base (2c82dd9) to head (bc938cc).

Files with missing lines	Patch %	Lines
lib/viewer.ts	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
+ Coverage   91.33%   91.39%   +0.06%     
==========================================
  Files          41       41              
  Lines        3532     3546      +14     
  Branches      816      819       +3     
==========================================
+ Hits         3226     3241      +15     
+ Misses        287      286       -1     
  Partials       19       19              

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[mejo-]

Current reference widgets allow switching to write mode and inline editing for some file types. At least Text (for Markdown/plaintext) and Richdocuments (for office documents) support it as far as I know. I'm not sure though whether we want to keep it as it often is very bad UX. Personally I'd prefer a button to open the previewed file full-screen in a overlay modal, read-write where supported.

[skjnldsv]

I reworded it so embedded is only a hint and Richdocuments can still decide for itself. For the files widget I'd do what you suggest: read-only inline, plus a button to open it in the viewer.