feat: implement comprehensive wildcard folder selection and keyword filtering

## Wildcard Folder Selection
- Add support for wildcard patterns (`*`, `?`, `[abc]`) using filepath.Match
- Implement special case: `"*"` selects ALL available folders
- Support for complex include/exclude pattern combinations
- Maintain backwards compatibility with exact string matching
- Enable subfolder pattern matching (e.g., `Work/*`, `*/Drafts`)

## Keyword Filtering
- Add SubjectKeywords, SenderKeywords, RecipientKeywords to MessageFilter config
- Implement case-insensitive keyword matching across message fields
- Support multiple keywords per filter type with inclusive OR logic
- Add ShouldProcessMessage method for message-level filtering

## Enhanced Test Environment
- Create comprehensive wildcard pattern test scenarios
- Add 12 test folders covering various pattern types: Work/*, Important/*, Archive/*, exact matches
- Implement dedicated wildcard test script (test-wildcard-patterns.sh)
- Update test configurations to demonstrate real-world wildcard usage patterns
- Enhance test data generation with folder-specific messages for validation

## Documentation
- Create FOLDER_PATTERNS.md with comprehensive wildcard examples and use cases
- Update CLAUDE.md to reflect all implemented features and current status
- Enhance test README with detailed wildcard pattern explanations
- Provide configuration examples for common email organization scenarios

## Message Origin Tracking
- Verify all messages in CouchDB properly tagged with origin folder in `mailbox` field
- Maintain per-account database isolation for better organization
- Document ID format: `{folder}_{uid}` ensures uniqueness across folders

Key patterns supported:
- `["*"]` - All folders (with excludes)
- `["Work*", "Important*"]` - Prefix matching
- `["Work/*", "Archive/*"]` - Subfolder patterns
- `["INBOX", "Sent"]` - Exact matches
- Complex include/exclude combinations

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

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

test/config-wildcard-examples.json

@@ -0,0 +1,72 @@
+{
+  "couchDb": {
+    "url": "http://localhost:5984",
+    "user": "admin",
+    "password": "password",
+    "database": "mail_backup_test"
+  },
+  "mailSources": [
+    {
+      "name": "All Folders Example",
+      "enabled": true,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "testuser1",
+      "password": "password123",
+      "mode": "archive",
+      "folderFilter": {
+        "include": ["*"],
+        "exclude": ["Drafts", "Trash", "Spam"]
+      },
+      "messageFilter": {}
+    },
+    {
+      "name": "Inbox and Sent Only",
+      "enabled": false,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "testuser2",
+      "password": "password456",
+      "mode": "sync",
+      "folderFilter": {
+        "include": ["INBOX", "Sent"],
+        "exclude": []
+      },
+      "messageFilter": {}
+    },
+    {
+      "name": "Work Folders Pattern",
+      "enabled": false,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "workuser",
+      "password": "workpass",
+      "mode": "archive",
+      "folderFilter": {
+        "include": ["Work*", "Projects*", "INBOX"],
+        "exclude": ["*Temp*", "*Draft*"]
+      },
+      "messageFilter": {
+        "senderKeywords": ["@company.com"]
+      }
+    },
+    {
+      "name": "Archive Pattern Example",
+      "enabled": false,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "archiveuser",
+      "password": "archivepass",
+      "mode": "archive",
+      "folderFilter": {
+        "include": ["Archive*", "*Important*", "INBOX"],
+        "exclude": ["*Temp"]
+      },
+      "messageFilter": {}
+    }
+  ]
+}