Data Recovery: a failed copy names the folder in use and how to change it

When a recovery copy could not be written, the status bar said only
"Couldn't save recovery data: <error>", and a copy that failed on the
background worker showed as "Saving recovery data for <title> failed:
<error>". Neither said which folder was in use or how to pick another.

The message keeps the error text (and the document's title when one
copy fails), then names the folder copies go to, says whether it is
the default recovery folder or the folder set in Preferences, and
names the setting that picks another folder: Preferences → File
Handling → Data Recovery → Folder (recoveryFolder). The recovery code
writes the same line to the log. The folder is the one in use when the
round started, so a preference changed while a copy runs does not
change the folder the message names.

A copy that fails now reports its own message, so the job's result is
Ok rather than an error, and the job no longer adds "<label> failed:
<error>" over it. A store the host installed (browser storage) has no
folder to name: its failed copy shows "Saving recovery data for
<title> failed: <error>", as a copy on the worker did before, also
when the copy was written without the worker.

- engine: recovery::folder returns the folder in use and whether the
  recoveryFolder preference names it; recovery::store uses it.
- Four @msg rows in each complete-message catalog, and in the bg, bs,
  da and id catalogs, which translate the old message.
- Control replies to a client confined to read roots show a recovery
  folder outside them, and the paths under it, by the folder's name, as
  they already show other paths outside the roots. That covers the
  folder in use and the last eight folders a failure message named.
- docs/target-app-parity.md: a line next to the Data Recovery Folder
  preference.

docs/target-app-parity.md

@@ -1540,6 +1540,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - `.vectorcraft` (lossless JSON, compressed, atomic saves, save down to v1), Save As with a format chooser, Save a Copy, Revert, templates, Data Recovery after a crash (browser tabs hold their copies with Web Locks too, so a background tab's copies are never taken for a crash's, and a tab repairs copies another removed when it resumes: #367), background save and export. The desktop app writes its warnings and errors (graphics adapter, a lost graphics device, panics the guard recovered from) to standard error and to `logs/vectorcraft.log` next to the preferences, the two previous runs kept and each file capped at 16 MiB, so a launch without a terminal still leaves something to attach to a bug report (#465). Native documents with a lot of path data (traced or generated art, in layers, symbols or patterns) are saved as compact JSON, about a fifth of the size; small documents stay pretty-printed (#1492). Saving to a network drive on macOS falls back to a plain fsync when the share refuses a full flush (#1625, part of #1620; not confirmed on a real share), and a save still succeeds when a folder refuses the temporary file, the rename over the document or copying permissions: the content is written elsewhere first and copied in, and a failed copy keeps the temporary file and names it (#1649, part of #1620). A document opened by a relative path finds its relatively linked images, and `vectorcraft-cli convert` lists what reading the input left out as `importWarnings` (#1606, fixes #1604).
     Preferences › File Handling › Data Recovery › Folder has a Choose… button on the desktop that picks the folder Data Recovery keeps its copies in ([Illustrator reference, Set Data Recovery preferences](https://web.archive.org/web/20210601162814/https://helpx.adobe.com/pdf/illustrator_reference.pdf)); a picked folder that Data Recovery cannot create its folders in or list is refused with a status message, and the field keeps its value. OK applies the same check to a folder typed into the field (VectorCraft's own rule): when it fails, the dialog stays open, the status bar says why and no preference changes; an empty field (the default folder) and the folder already saved are not checked. Preferences › Plug-ins & Scratch Disks is where additional plug-ins folders are set ([Illustrator preferences help](https://helpx.adobe.com/illustrator/desktop/get-started/preferences-and-settings/set-app-preferences.html)); its Additional Plug-ins Folder has a Choose… button on the desktop as well, VectorCraft's own, matching Data Recovery's, and VectorCraft's own rule refuses a picked or typed folder whose contents can't be listed. Additional Fonts Folder and Templates Folder stay typed only.
     `prefs.set` applies the dialog's check to `recoveryFolder` and `pluginsFolder` for every caller (VectorCraft's own rule): a folder that fails returns an error and no preference changes; an empty value and the folder already saved are not checked.
+    When a Data Recovery copy can't be written, the status bar and the log show the error (with the document's title when one document's copy fails), the folder copies go to, whether that is the default recovery folder or the folder set in Preferences, and the preference that picks another folder (`recoveryFolder`; VectorCraft's own message).
     A name typed in the desktop save panel without the file type's extension gets it, and when a file of that name exists, which the panel did not ask about, Save, Save As, Save a Copy, Save as Template, the exports and every other write through a save panel ask before replacing it (dialog `replaceFile`; Cancel writes nothing).
     A name returned by the desktop save panel with the extension of a format the save does not write has it replaced with the saved format's: Save As of a new document saves `card.ait` and `card.png` as `card.vectorcraft`, and Save as Template saves `card.pdf` as `card.vctemplate`. An extension that names no format stays part of the name (`notes.v2` is saved as `notes.v2.vectorcraft`).
     Export As, Save as PDF, Save for Office Documents, Save for Web, Text Export Options, print to file, the presets' Export… and the library, swatch and Document Info saves replace a typed extension of another format than the one they write in the same way: Export As PNG writes `card.svg` as `card.png`, Save as PDF writes `card.png` as `card.pdf`, and `document.exportSelection` with a `format` suggests and adds that format's extension. An extension of the written format stays as typed in any letter case (`photo.JPEG` for JPEG), Export As SVG writes `art.svgz` compressed, and `document.exportSelection` without a `format` writes the format the typed extension names (`tests/saveext.rs`).