Fix Docker build failure: add git to builder stage (#960)

The Docker browser build fails because yarn cannot install
`@electron/node-gyp` from GitHub without git.

## Changes

- Add git to builder stage in `Dockerfile.browser`
- Git is excluded from final production image via multi-stage build

## Technical Details

The `yarn.lock` contains a git-based dependency:
```
"@electron/node-gyp@https://github.com/electron/node-gyp#06b29aafb7708acef8b3669835c8a7857ebc92d2"
```

Alpine Linux base image lacks git by default. Installing it only in the
builder stage (where `yarn install` runs) resolves the build failure
without affecting production image size.

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<details>

<summary>Original prompt</summary>

> fix
https://github.com/thomasnordquist/MQTT-Explorer/actions/runs/20442696351/job/58739200179


</details>



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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: thomasnordquist <7721625+thomasnordquist@users.noreply.github.com>

Dockerfile.browser

@@ -4,6 +4,9 @@ FROM node:24-alpine AS builder
 
 WORKDIR /build
 
+# Install git (required for git-based dependencies like @electron/node-gyp)
+RUN apk add --no-cache git
+
 # Copy package files for dependency installation
 COPY package.json yarn.lock ./
 COPY app/package.json ./app/