fix: Update ISO download URLs for Debian 13 availability

Debian 13 (trixie) doesn't have stable release ISO yet.
Updated build-iso.sh to:
- Try daily builds first (most current trixie)
- Fallback to testing branch
- Final fallback to current (sid) if needed

This should resolve ISO download failures.

💘 Generated with Crush

Assisted-by: Gemini 2.5 Flash via Crush <crush@charm.land>

build-iso.sh

@@ -35,14 +35,18 @@ docker run --rm \
         echo "Downloading Debian 13 Trixie Netboot ISO..."
         cd /build/iso-tmp
 
-        # Download Debian 13 trixie netboot ISO (smaller, will download packages during install)
+        # Use daily builds for Debian 13 (trixie) since stable ISO not available yet
         wget -q --show-progress \
             -O debian-trixie-amd64-netinst.iso \
-            https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-13.0.0-amd64-netinst.iso || \
+            https://cdimage.debian.org/debian-cd/daily-builds/amd64/iso-cd/debian-daily-amd64-netinst.iso || \
-        # Fallback to testing if release not available
+        # Fallback to testing
         wget -q --show-progress \
             -O debian-trixie-amd64-netinst.iso \
-            https://cdimage.debian.org/debian-cd/testing/amd64/iso-cd/debian-testing-amd64-netinst.iso
+            https://cdimage.debian.org/debian-cd/testing/amd64/iso-cd/debian-testing-amd64-netinst.iso || \
+        # Fallback to current stable (bookworm/sid)
+        wget -q --show-progress \
+            -O debian-trixie-amd64-netinst.iso \
+            https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-sid-amd64-netinst.iso
 
         echo ""
         echo "✅ ISO downloaded"