fix(editor): polish slash commands and align split documents

[ThisIs-Developer]

The slash menu now matches the application's dropdown styling and keeps the typing line visible. This also completes the command audit: media uploads stay in the originating editor, the documented Reference command is available, and Date/time matches the toolbar.

• Reuse the shared menu surface, typography, spacing, Lucide icons, and heading symbols in both themes.
• Fit the menu above or below the caret and scroll its contents internally, including after filtering and viewport changes.
• Carry the target editor through media upload, persistence, and rollback. Cancel insertion when the target document changes during consent or upload, and save restored content after a persistence failure.
• Add Reference using the existing chooser, including automatic numbering in either editor. Share the toolbar's date/time formatter.
• Align both documents in split view using the shared outer pane padding and matching preview backgrounds. Show the same typing/paste/import placeholder in both empty editors.
• Remove the secondary document's blue focus outline in Edit and Preview.
• Synchronize desktop resources and update the feature and usage documentation.

Validation:

• npm run build and all 22 unit tests passed.
• Verified 66 slash-menu and command-action cases across Chromium and Firefox, covering all 22 commands in both editor panes, including placeholder selections, all five alerts, table/diagram/media choosers, references, persisted uploads, cancellation, save recovery, keyboard navigation, accessibility, RTL, and constrained viewport placement.
• The corrected upload/persistence/recovery subset passed all 8 cases in a focused rerun. Media upload tests intercept the API; no files are uploaded externally.
• Split-view behavior tests passed in Chromium and Firefox. Browser measurements and screenshots verified matching editor/preview top offsets and backgrounds at 1280px and 375px in both themes, plus both empty-editor placeholders and the absence of a focus outline in the secondary Editor and Preview.
• node desktop-app/prepare.js, JavaScript syntax checks, and git diff --check passed.

WebKit could not run on this Windows machine: the installed runtime fails with spawn UNKNOWN, and a fresh task-local installation reports missing icutu77.dll.

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Project	Deployment	Actions	Updated
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