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/go.mod

@@ -4,11 +4,11 @@ go 1.24.4
 
 require (
 	github.com/emersion/go-imap/v2 v2.0.0-beta.5
+	github.com/emersion/go-message v0.18.1
 	github.com/go-kivik/kivik/v4 v4.4.0
 )
 
 require (
-	github.com/emersion/go-message v0.18.1 // indirect
 	github.com/emersion/go-sasl v0.0.0-20231106173351-e73c9f7bad43 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	golang.org/x/net v0.25.0 // indirect