feat(permalinks): point Share links/QR at to.healthchat.ch

Add a config-driven matrix_to_url option so Share dialog links and QR
codes resolve against a matrix.to-compatible resolver instead of the
hardcoded public matrix.to. MatrixToPermalinkConstructor now reads the
base URL lazily from config (default https://matrix.to), keeping the
matrix.to URL format. HealthChat configs point it at the in-house fork
https://to.healthchat.ch and drop the conflicting permalink_prefix
(which routes to the Element-format constructor a matrix.to fork can't
parse).

apps/web/config.json

@@ -35,7 +35,7 @@
         "preferred_domain": "jitsi.healthchat.ch"
     },
     "logout_redirect_url": "",
-    "permalink_prefix": "https://matrix.to",
+    "matrix_to_url": "https://to.healthchat.ch",
     "room_directory": {
         "servers": [
             "matrix.org"