feat: `.gradle/` directory and `build.gradle` files are committed — these are Android build artifacts that should be gitignored, causing unnecessary repository bloat and platform-specific conflicts

## 🐛 Problem Statement

The OpSo repository contains committed Android build artifacts:

- `.gradle/` directory — Gradle build cache and wrapper state (machine-specific, binary files)
- `build.gradle` at the root — This appears to be committed separately from `android/build.gradle`
- `local.properties` — Contains the **local SDK path** (e.g., `/Users/developer/Library/Android/sdk`) which is 100% machine-specific and must never be in version control

Specifically, `local.properties` leaks:
- The original developer's home directory name (e.g., `sdk.dir=/Users/andoriya/Library/Android/sdk`)
- Operating system (Linux vs macOS vs Windows path format)

Every contributor who clones the repo will have a different Android SDK location and must overwrite `local.properties` manually — a confusing step especially for beginners.

## Fix Required

```bash
# Step 1: Remove from git tracking
git rm --cached local.properties
git rm --cached -r .gradle/
# Note: Do NOT remove android/build.gradle — that's part of the project

# Step 2: Add proper Flutter .gitignore entries
cat >> .gitignore << 'EOF'
# Android build files
local.properties
.gradle/
android/.gradle/

# Flutter build output
build/
.dart_tool/

# iOS build
ios/.symlinks/
ios/Pods/
*.lock (for Pods)

# macOS
macos/.symlinks/
EOF

git commit -m "chore: remove machine-specific build artifacts from version control"
```

**Also add to `README.md`** a note that contributors need to create their own `local.properties`:

```markdown
## Setup Note for Android Development
After cloning, create `local.properties` in the root with your SDK path:
```
sdk.dir=/path/to/your/android/sdk
This file is gitignored and must be created locally.

## Files to Modify

| File | Change |
|---|---|
| `.gradle/` | Remove from git tracking |
| `local.properties` | Remove from git tracking (machine-specific SDK path) |
| `.gitignore` | Add `.gradle/`, `local.properties`, Flutter build patterns |
| `README.md` | Add "Setup Note for Android Development" |

**Suggested labels:** `chore`, `good first issue`, `documentation`

I would like to work on this. Could you please assign it to me?

Review .gitignore, the tracked .gradle/ directory, local.properties, and README.md first; confirm which Android and Flutter artifacts are currently committed. Remove only machine-specific artifacts from tracking, preserve android/build.gradle, add the requested ignore patterns, and document local.properties setup in README.md. Verify the final git status and that the repository no longer tracks those local files.

由索引模型根据 Issue 内容生成。