add standalone insert citation dialog for hosts without an editor

[kevinushey]

This makes the visual editor's Insert Citation dialog usable without an editor instance, so RStudio can offer it in its plain-text source editor (rstudio/rstudio#7876, a long-standing request).

The dialog itself never needed the ProseMirror document. It only read the YAML front matter to find bibliography:, references: and zotero:. InsertCitationCommand then applied the result with a transaction: the [@key] text and, when a new bibliography file was created, a bibliography: line in the front matter. This PR separates those two seams from the rest.

Changes

• BibliographyManager and its providers accept either a document or plain YAML blocks (YamlBlockSource). Existing callers keep passing the doc and are unchanged.
• ensureSourcesInBibliography is split. The new writeSourcesToBibliography does the server writes and the Better BibTeX warning, and the wrapper still adds bibliography: to the doc in the same transaction.
• showInsertCitationDialog takes a YamlBlockSource. The unused doc parameter is removed from the panel and the bibliography source panel.
• A new InsertCitationDialog class (editor/editor-citation.ts) is exported from editor and from the panmirror bundle. show({ yaml, theme?, selectionKey? }) returns { citationIds, intextCitationStyle, bibliographyFile?, selectionKey? }. The host inserts the citation text and declares bibliographyFile in its front matter when set.
• editorUIWithDefaultImages is extracted from Editor.create. This also corrects the omni_insert and citations merges, which spread the whole images object instead of the nested one. editor-ui passes the full defaults, so its images are unchanged.
• ensureTheme applies the default theme CSS when no editor has applied a theme yet, since the panel relies on the pm-* color classes.

Verification

tsc --noEmit passes for apps/panmirror and apps/vscode-editor, and eslint passes on the changed files.

Standalone dialog. I loaded the built bundle in headless Chromium with a stub host and server and drove the dialog:

Scenario	Result	Bibliography write
Existing local source, doc has no bibliography:	citationIds: ["smith2020"], bibliographyFile: "references.bib"	none, already present
From DOI, doc declares refs.bib	citationIds: ["rivera2021"], no bibliographyFile	BibTeX entry appended to refs.bib
Cancel	null	none

Visual editor, before and after. In an RStudio development server, I inserted an R Package citation from the visual editor's Insert Citation dialog in a new .md document. I ran it once with the current bundle and once with this branch's bundle served in its place:

	[@knitr] inserted	bibliography: references.bib added	references.bib has the knitr entry
before	yes	yes	yes
after	yes	yes	yes

Notes

• There's no apps/vscode/CHANGELOG.md entry, because nothing changes for VS Code or Positron users. The new export is for RStudio. Happy to add one if you'd prefer.
• packages/editor-collab and packages/editor-codemirror type-check editor/src without JSX enabled, so their tsc already reports TS6142 for the .tsx modules it reaches. The new file adds one more of the same, and neither package has a tsc script or CI step.

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