Chore: add CodeQL and community conduct

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Security vulnerability
+    url: https://github.com/lsj1206/PixConvert/security/policy
+    about: Please review the security policy before reporting sensitive issues.
+  - name: GitHub Releases
+    url: https://github.com/lsj1206/PixConvert/releases
+    about: Download the latest release package before reporting issues fixed in newer versions.

.github/workflows/codeql.yml

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+name: CodeQL
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+  schedule:
+    - cron: "30 18 * * 1"
+
+permissions:
+  actions: read
+  contents: read
+  packages: read
+  security-events: write
+
+jobs:
+  analyze:
+    name: Analyze (csharp)
+    runs-on: windows-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: 10.0.x
+
+      - name: Restore
+        run: dotnet restore PixConvert.sln
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v4
+        with:
+          languages: csharp
+          build-mode: manual
+
+      - name: Build
+        run: dotnet build PixConvert.sln -c Release --no-restore -v minimal -m:1
+
+      - name: Perform CodeQL Analysis
+        uses: github/codeql-action/analyze@v4
+        with:
+          category: "/language:csharp"

CODE_OF_CONDUCT.md

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+# Code of Conduct
+
+## Our Standards
+
+PixConvert aims to be a respectful and practical open source project. Contributors, users, and maintainers are expected to communicate clearly, assume good intent, and focus on improving the project.
+
+Examples of encouraged behavior include:
+
+- Using welcoming and inclusive language
+- Being respectful of different viewpoints and experience levels
+- Giving and receiving constructive technical feedback
+- Keeping discussions focused on the project and user impact
+- Acknowledging mistakes and correcting them when needed
+
+Examples of unacceptable behavior include:
+
+- Harassment, insults, or personal attacks
+- Discriminatory language or conduct
+- Public sharing of private information without permission
+- Repeated off-topic or disruptive comments
+- Any behavior that makes participation unsafe or hostile
+
+## Scope
+
+This Code of Conduct applies to project spaces including issues, pull requests, discussions, release comments, and other public project-related communication.
+
+## Enforcement
+
+Maintainers may remove comments, close discussions, block users, or take other reasonable action when behavior violates this Code of Conduct.
+
+If you need to report conduct concerns, contact the maintainer through GitHub profile contact methods. Do not include private personal information in public issues. If the report also involves a security vulnerability, follow the process in `SECURITY.md` instead.
+
+## Attribution
+
+This policy is inspired by common open source community standards, including the Contributor Covenant.