Also report free flash in gh comment

.github/workflows/build.yml

@@ -128,10 +128,14 @@ jobs:
           dfu_size_new=$(du --apparent-size -B 1 artifacts/flipper-z-${TARGET}-full-*.dfu | cut -f1)
           dfu_size_dev=$(du --apparent-size -B 1 dev.dfu | cut -f1)
           dfu_size_diff=$((dfu_size_new - dfu_size_dev))
+          dfu_free_hex=$(grep free_flash -m 1 build/latest/firmware.elf.map | rev | cut -d' ' -f1 | rev)
+          dfu_free_min=$((2 * 4 * 1024))
           DFU_SIZE=$(echo ${dfu_size_new}B | sed -r 's/^([0-9]+)([0-9]{2})([0-9])B/\1.\2K/')
           DFU_DIFF=$(echo ${dfu_size_diff}B | sed -r 's/^(-?[0-9]+)([0-9]{2})([0-9])B/\1.\2K/' | sed -r 's/^([^-])/+\1/')
+          DFU_FREE=$(echo $((dfu_free_hex - dfu_free_min))B | sed -r 's/^(-?[0-9]+)([0-9]{2})([0-9])B/\1.\2K/' | sed -r 's/^([^-])/+\1/')
           echo "DFU_SIZE=$DFU_SIZE" >> $GITHUB_ENV
           echo "DFU_DIFF=$DFU_DIFF" >> $GITHUB_ENV
+          echo "DFU_FREE=$DFU_FREE" >> $GITHUB_ENV
 
       - name: "Upload artifacts to update server"
         env:
@@ -169,7 +173,7 @@ jobs:
             **Compiled ${{ matrix.target }} firmware for commit `${{steps.names.outputs.commit_sha}}`:**
             - [☁️ Flipper Lab/App](https://lab.flipper.net/?url=${{secrets.INDEXER_URL}}/builds/firmware/${{steps.names.outputs.branch_name}}/flipper-z-${{steps.names.outputs.default_target}}-update-${{steps.names.outputs.suffix}}.tgz&channel=mntm-${{steps.names.outputs.branch_name}}&version=${{steps.names.outputs.commit_sha}})
             - [📦 qFlipper Package](${{secrets.INDEXER_URL}}/builds/firmware/${{steps.names.outputs.branch_name}}/flipper-z-${{steps.names.outputs.default_target}}-update-${{steps.names.outputs.suffix}}.tgz)
-            - DFU Size: `${{ env.DFU_SIZE }}` (`${{ env.DFU_DIFF }}` from dev)
+            - DFU Size: `${{ env.DFU_SIZE }}` (`${{ env.DFU_DIFF }}` from dev), `${{ env.DFU_FREE }}` free flash
           edit-mode: replace
 
       - name: Send devbuild webhook