refactor: remove webmail interface, focus on core mail storage functionality

- Remove obsolete CouchDB design documents (webmail.json, dashboard.json)
- Clean up webmail-related code from couch/couch.go (WebmailViews, CreateWebmailViews, etc.)
- Update documentation to focus on core mail-to-CouchDB storage functionality
- Add Future Plans section describing planned webmail viewer as separate component
- Apply go fmt formatting and ensure code quality standards
- Update test documentation to show raw CouchDB API access patterns
- Remove compiled binary from repository

This refactor simplifies the codebase to focus on its core purpose: efficiently
backing up emails from IMAP to CouchDB. The webmail interface will be developed
as a separate, optional component to maintain clean separation of concerns.

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

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

go/config/config.go

@@ -40,7 +40,7 @@ type FolderFilter struct {
 type MessageFilter struct {
 	Since             string   `json:"since,omitempty"`
 	SubjectKeywords   []string `json:"subjectKeywords,omitempty"`   // Filter by keywords in subject
-	SenderKeywords    []string `json:"senderKeywords,omitempty"`    // Filter by keywords in sender addresses  
+	SenderKeywords    []string `json:"senderKeywords,omitempty"`    // Filter by keywords in sender addresses
 	RecipientKeywords []string `json:"recipientKeywords,omitempty"` // Filter by keywords in recipient addresses
 }