feat(ci): Add Raspberry Pi 400 to firmware build matrix

- Add rpi-400 device using bcm27xx/bcm2711 target (same as Pi 4)
- Update release notes to include Pi 400 in supported devices

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/build-firmware-imagebuilder.yml

@@ -117,6 +117,15 @@ jobs:
               "description": "Raspberry Pi 5",
               "extra_packages": ""
             },
+            {
+              "name": "rpi-400",
+              "target": "bcm27xx",
+              "subtarget": "bcm2711",
+              "profile": "rpi-4",
+              "platform": "raspberry-pi",
+              "description": "Raspberry Pi 400",
+              "extra_packages": ""
+            },
             {
               "name": "nanopi-r4s",
               "target": "rockchip",
@@ -722,6 +731,7 @@ jobs:
           ### Raspberry Pi
           - Raspberry Pi 3B/3B+
           - Raspberry Pi 4B
+          - Raspberry Pi 400
           - Raspberry Pi 5
 
           ### NanoPi / FriendlyARM