refactor: reorganize project structure with src/ directory

Move all Python source code into src/ directory for better project organization:
- All .py files moved to src/
- Added src/__init__.py to make it a proper Python package
- Updated mcp.json entry path to src/mail_mcp_server.py
- Updated README.md and README_zh.md configuration examples

Benefits:
- Cleaner root directory (documentation and config only)
- Follows Python project conventions
- Easier to maintain as project grows

Files moved:
- mail_mcp_server.py
- parse_email.py
- extract_attachments.py
- cleanup_attachments.py
- get_email_path.py
- get_thread_paths.py

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: fatbobman

README.md

@@ -76,7 +76,7 @@ pip3 install mcp
        "mail": {
          "command": "python3",
          "args": [
-           "/path/to/mail-mcp-bridge/mail_mcp_server.py"
+           "/path/to/mail-mcp-bridge/src/mail_mcp_server.py"
          ]
        }
      }
@@ -93,7 +93,7 @@ pip3 install mcp
        "mail": {
          "command": "python3",
          "args": [
-           "/path/to/mail-mcp-bridge/mail_mcp_server.py"
+           "/path/to/mail-mcp-bridge/src/mail_mcp_server.py"
          ],
          "env": {
            "MAIL_ATTACHMENT_PATH": "/tmp"

README_zh.md

@@ -78,7 +78,7 @@ pip3 install mcp
        "mail": {
          "command": "python3",
          "args": [
-           "/path/to/mail-mcp-bridge/mail_mcp_server.py"
+           "/path/to/mail-mcp-bridge/src/mail_mcp_server.py"
          ]
        }
      }

mcp.json

@@ -8,7 +8,7 @@
   "runtime": {
     "type": "python",
     "command": "python3",
-    "entry": "mail_mcp_server.py"
+    "entry": "src/mail_mcp_server.py"
   },
   "tools": [
     "get_email_path",