feat: implement server-side IMAP LIST and SEARCH filtering in Rust

Add server-side folder filtering using IMAP LIST patterns and enhance
message filtering to use IMAP SEARCH with keyword filters when available.

Key improvements:
- Add list_filtered_mailboxes() method using IMAP LIST with patterns
- Use server-side filtering instead of client-side folder filtering
- Enhance message search to use IMAP SEARCH for subject/sender keywords
- Add has_keyword_filters() method to MessageFilter
- Reduce network traffic by leveraging IMAP server capabilities
- Remove dependency on client-side filter_folders function

This achieves full feature parity with the updated Go implementation
and ensures both versions use IMAP standards optimally.

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

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

rust/src/config.rs

@@ -77,6 +77,15 @@ pub struct MessageFilter {
     pub recipient_keywords: Vec<String>,
 }
 
+impl MessageFilter {
+    /// Check if this filter has any keyword-based filters that can use IMAP SEARCH
+    pub fn has_keyword_filters(&self) -> bool {
+        !self.subject_keywords.is_empty() || !self.sender_keywords.is_empty()
+        // Note: recipient_keywords not included as IMAP SEARCH doesn't have a TO field search
+        // that works reliably across all IMAP servers
+    }
+}
+
 fn default_mode() -> String {
     "archive".to_string()
 }