plugin-app: substitute path params in app/routes redirect targets

The app/routes redirect config now performs the same :param and *
substitution that the legacy Redirect component did before navigating.
Named params captured by the `from` pattern are replaced in the `to`
string, enabling redirects like /users/:userId → /profile/:userId and
/old-docs → /docs/* (with splat forwarding).

Adds tests for both named-param and splat substitution.

Signed-off-by: Patrik Oldsberg <rugvip@backstage.io>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

.changeset/app-routes-param-redirect-substitution.md

@@ -0,0 +1,17 @@
+---
+'@backstage/plugin-app': patch
+---
+
+The `app/routes` redirect config now supports path parameter substitution in the `to` target. Named params (`:userId`) and splat params (`*`) captured by the `from` path are replaced in the `to` string before navigating, making it possible to express redirects like:
+
+```yaml
+app:
+  extensions:
+    - app/routes:
+        config:
+          redirects:
+            - from: /users/:userId
+              to: /profile/:userId
+            - from: /old-docs
+              to: /docs/*
+```