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

custom_components/houseplan/websocket_api.py

@@ -13,7 +13,7 @@ from homeassistant.core import HomeAssistant, callback
 
 from .const import (
     CONF_ADMIN_ONLY, DEFAULT_CONFIG,
-    PLANS_DIR, PLANS_URL,
+    CONTENT_URL, PLANS_DIR, PLANS_URL,
 )
 from .store import HouseplanData, get_data, get_entry
 from .validation import (
@@ -290,5 +290,5 @@ async def ws_plan_set(hass: HomeAssistant, connection, msg: dict[str, Any]) -> N
 
     mtime = await hass.async_add_executor_job(_write)
     connection.send_result(
-        msg["id"], {"ok": True, "url": f"{PLANS_URL}/{space_id}.{msg['ext']}?v={mtime}"}
+        msg["id"], {"ok": True, "url": f"{CONTENT_URL}/plans/_/{space_id}.{msg['ext']}?v={mtime}"}
     )