ci: drop coveralls, add codecov config, change badge

.github/workflows/coverage.yaml

@@ -9,8 +9,6 @@ env:
 jobs:
   coverage:
     runs-on: ubuntu-latest
-    env:
-      COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}
     steps:
       - name: Checkout code
         uses: actions/checkout@v5
@@ -54,35 +52,5 @@ jobs:
         uses: codecov/codecov-action@v5
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
-          files: lcov.info
-
-      - name: Download Coveralls CLI
-        if: ${{ env.COVERALLS_REPO_TOKEN != '' }}
-        run: |
-          # use GitHub Releases URL instead of coveralls.io because they can't maintain their own files; it 404s
-          curl -L https://github.com/coverallsapp/coverage-reporter/releases/download/v0.6.15/coveralls-linux-x86_64.tar.gz | tar -xz -C /usr/local/bin
-
-      - name: Upload coverage to Coveralls
-        if: ${{ env.COVERALLS_REPO_TOKEN != '' }}
-        run: |
-          if [ ! -f "lcov.info" ]; then
-            echo "Error: lcov.info file not found. Coverage generation may have failed."
-            exit 1
-          fi
-
-          for i in {1..10}; do
-            echo "Attempt $i: Uploading coverage to Coveralls..."
-            if coveralls -n report lcov.info; then
-              echo "Successfully uploaded coverage report."
-              exit 0
-            fi
-
-            if [ $i -lt 10 ]; then
-              delay=$((2**i))
-              echo "Attempt $i failed. Retrying in $delay seconds..."
-              sleep $delay
-            fi
-          done
-
-          echo "Failed to upload coverage report after 10 attempts."
-          exit 1
+          files: ./lcov.info
+          disable_search: true