fix: Enable push on MIPS by switching web-push client

- Use hyper-client to avoid isahc/coarsetime (AtomicU64 on MIPS)
- Keep push notifications enabled for MIPS builds
- Update CI MIPS build to use default features

.github/workflows/build-mips.yml

@@ -55,7 +55,7 @@ jobs:
         CROSS_NO_WARNINGS: 0
       run: |
         cd backend
-        cross build --target mips-unknown-linux-musl --release --no-default-features -Z build-std=std,panic_abort
+        cross build --target mips-unknown-linux-musl --release -Z build-std=std,panic_abort
 
     - name: Debug - List Files
       run: |