web: fix red-team breaks in restore, drop-open, and plot paths

Three attack agents (state-machine tracing, executable input fuzzing,
browser-API review) found two breaks and several degradations; all are
fixed here, with the fuzzer's 22 tests kept as regression suites.

Breaks:
- Untitled tabs (and dot-names like ".dxf") autosave as DXF, but the
  restore name defaulted to .dwg — the worker dispatches its reader by
  the name's extension, so the only copy of unsaved work could never
  be restored and was re-offered forever. recovered_name now uses the
  worker's rsplit('.') convention and defaults extensionless names to
  dxf, matching new-document serialization.
- " (recovered)" stacked one suffix per crash-restore cycle; the
  marker is now stripped idempotently before appending.
- Opening a file (Ctrl+O / OPEN / automation) while a web open was in
  flight clobbered it: the in-flight open's bytes were lost and every
  queued restore prompt stalled for the session. on_open_file now
  refuses while an open is running (web only).

Hardening:
- Escape on the Recover Drawing modal defers instead of deleting the
  only snapshot; the Discard button still removes it.
- A cap-rejected restore enqueue warns in-app and re-offers the next
  snapshot; the drop queue cap rises to 32.
- Exiting mid-round no longer races snapshot removals (deferrals are
  drained, the sweep skips while a round is in flight); closing a tab
  with "Don't save" now also removes its web snapshot; the snapshot
  key derivation is shared via web_autosave_key.
- The plot-preview overlay no longer leaks a full PDF copy per preview
  (the Download closure lives in a replaceable registry).
- A corrupt autosave index.json fails the round/boot instead of
  silently resetting every row; plot pages with non-finite or non-
  positive sheet dimensions error instead of emitting a NaN MediaBox.
- The drop filter accepts dot-less "DWG"/"dxf" names the worker
  parses (same rsplit convention on both gates).

(cherry picked from commit 3eab00bfbf)

Ported onto main (#1717 review): the five standalone tests/*_redteam.rs
files stay out — no test target builds them and they exercised copies of
the functions rather than the code itself.

locales/ru-RU/opencadstudio.ftl

@@ -281,6 +281,8 @@ common =
     .autosave-copy-could-not-be-stored = Не удалось сохранить копию автосохранения: __ocs_fmt_0__
     .restored-from-the-autosave-copy = «__ocs_fmt_0__» восстановлен из копии автосохранения
     .autosave-copy-could-not-be-read = Не удалось прочитать копию автосохранения: __ocs_fmt_0__
+    .an-open-is-already-in-progress = Открытие файла уже выполняется.
+    .autosave-copy-could-not-be-queued-too-many-pending-opens = Не удалось поставить копию автосохранения в очередь: слишком много ожидающих открытий
     .jogged-radius = Радиус с изломом
     .none-use-shortcuts-set-key-command = (нет — используйте: SHORTCUTS SET <key> <command>)
     .alignment-indented-label-title-case = Выравнивание: