v1.44.7: plan backgrounds never displayed (regression from v1.44.5)

The card signs content urls because a browser cannot authenticate an <image
href>. But _display() was called inside _buildModel(), and the space model is
memoized on the config fingerprint — so the UNSIGNED url froze in the cache and
the signature, which did arrive, never reached the element. The plan never
loaded, and the browser kept hitting the unsigned path: 401, which Home
Assistant reports as a failed login attempt from the viewer's own IP (that is
how the owner spotted it). PDF links were unaffected: they already resolved at
render time.

- _buildModel() keeps the raw plan_url; the render pass calls _display().
- _display() returns '' for an unsigned content url instead of the plain path,
  and the <image> is not emitted at all until the signature lands — no 401, no
  spurious login-attempt warning.
- _resign() replaces 'drop everything and re-request': the previous urls are
  kept until the new ones arrive, so a wall tablet never blanks.
- demo/smoke_plan_signed.mjs: reproduces on v1.44.6 (href stays ?v=..., never
  ?authSig=), passes here. TESTING.md row added.
- docs: CHANGELOG.md + CHANGELOG.ru.md + STATUS.md.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "houseplan-card",
-  "version": "1.44.6",
+  "version": "1.44.7",
   "description": "Interactive house plan Lovelace card for Home Assistant",
   "license": "MIT",
   "type": "module",