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/config.rs

@@ -107,6 +107,7 @@ impl MailSource {
 pub struct CommandLineArgs {
     pub config_path: Option<String>,
     pub max_messages: Option<u32>,
+    pub dry_run: bool,
     pub generate_bash_completion: bool,
     pub help: bool,
 }
@@ -283,6 +284,7 @@ mod tests {
         let args = CommandLineArgs {
             config_path: None,
             max_messages: None,
+            dry_run: false,
             generate_bash_completion: false,
             help: false,
         };