feat(prototype): Edit in the IDE opens the file, not just the project

[AndrejK666]

What it did

The button handed the IDE a project and left the person to find the document
themselves — in a repository of five thousand files, with the path sitting right
there in the row they had just clicked.

What was already there

Almost all of it, unused:

• the IDE's portal bridge has handled studio.openInEditor since it was
  written, and OpenInEditorFrontendController resolves a repo-relative path
  against every workspace root and each root's children — because a repository
  is cloned to /workspace/<name> — warning by name when the file is not
  checked out in that session;
• openInStudioEditor has been sitting in App.tsx with no callers at
  all;
• a binding's path is repo-relative, which is exactly the shape that opener
  wants.

So this connects three things that were each waiting for the other two.

The only real work: timing

Opening a document means starting a session when none is running, and a message
posted at an iframe that has not loaded its bridge yet is simply lost — the same
failure the studio.init retry already exists for ("the iframe's first load
events are the session gate's redirect/splash pages").

The path is therefore both posted immediately, for a session already up,
and remembered for the bridge's first sign of life — which the message
listener already watches for, to end that retry. One extra branch there drains
it.

The slot is module-scoped rather than a prop threaded from the shell down
through the project screen: one browser tab opens one file at a time, and the
listener that drains it lives several components above the button that fills it.

tsc -b and the 41 prototype tests clean. Frontend only; no change to the Theia
extension, which could already do this.

[coderabbitai]

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