fix v1.43.0: external audit P0 — data loss, split geometry, auth, dialog zombies

L2 (silent data loss): debounce gains flush()/pending(); _reloadConfigOnly
flushes a pending write and defers while one is in flight; conflict path
forces; failed reload now toasts instead of an empty catch; teardown flushes.

G1 (split corruption): same-edge cuts carve the niche properly instead of
walking the outline twice; partition invariant (parts sum to the original)
rejects anything else; +1 unit test covering 5 niche shapes and both legacy
cut shapes.

B1 (unauthenticated content): plans and marker files move to
HouseplanContentView (/api/houseplan/content/..., requires_auth); only the
card bundle stays static; contentUrl() rewrites legacy URLs on read (no
storage migration); repairs.py accepts both prefixes; +1 unit test.

L3 (dialog zombies): all four save catch-blocks guard against a closed
dialog; the card no longer blanks when a save fails after Esc.

smokes: smoke_save_race, smoke_dialog_zombie; docs (TESTING/CHANGELOG/
ARCHITECTURE incl. the optimistic-UI note) same-commit

src/i18n/ru.json

@@ -322,5 +322,6 @@
   "room.sizes_section": "Размеры шрифтов",
   "room.name_scale": "Размер названия",
   "room.label_scale": "Размер подписей",
-  "preview.room_name": "Гостиная"
+  "preview.room_name": "Гостиная",
+  "toast.cfg_reload_failed": "Не удалось перечитать план с сервера: {err}"
 }