feat: add --dry-run mode to Rust implementation

Add comprehensive dry-run functionality to the Rust implementation that allows
users to test their configuration without making any changes to CouchDB:

- Added --dry-run/-n command line flag with clap argument parsing
- Extended CommandLineArgs struct with dry_run field
- Updated bash completion script to include new flag
- Comprehensive dry-run logic throughout sync coordinator:
  - Skip database creation with informative logging
  - Skip sync metadata retrieval and use config fallback
  - Skip deleted message handling in sync mode
  - Skip message and attachment storage with detailed simulation
  - Skip sync metadata updates with summary information
- Enhanced summary output to clearly indicate dry-run vs normal mode
- Updated all tests to include new dry_run field
- Maintains all IMAP operations for realistic mail discovery testing

This brings the Rust implementation to feature parity with the Go version
for safe configuration testing as identified in ANALYSIS.md.

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

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

rust/src/main.rs

@@ -36,6 +36,10 @@ async fn run(args: mail2couch::config::CommandLineArgs) -> Result<()> {
         info!("Maximum messages per mailbox: unlimited");
     }
 
+    if args.dry_run {
+        info!("🔍 DRY-RUN MODE: No changes will be made to CouchDB");
+    }
+
     // Display configuration summary
     print_config_summary(&config);