feat(cli): add interactive email import tool for EML, MBOX, and Thunderbird

- Implement `bichonctl` interactive CLI using `dialoguer`.
- Support recursive EML directory scanning with folder structure preservation.
- Support single MBOX file streaming import.
- Support Thunderbird profile import with automatic `.sbd` hierarchy detection.
- Add batch processing (Base64 encoding & batch API requests) for improved performance.

src/modules/cli/mbox/gmail.rs

@@ -0,0 +1,43 @@
+use std::collections::HashSet;
+
+pub fn determine_folder(labels_raw: &str) -> String {
+    let mut status_blacklist = HashSet::new();
+    status_blacklist.insert("Opened");
+    status_blacklist.insert("Unread");
+    status_blacklist.insert("Archived");
+
+    let all_labels: Vec<&str> = labels_raw
+        .split(',')
+        .map(|s| s.trim())
+        .filter(|s| !s.is_empty())
+        .collect();
+
+    if all_labels.is_empty() {
+        return "Unknown".to_string();
+    }
+
+    let filtered: Vec<&str> = all_labels
+        .iter()
+        .filter(|&&l| !status_blacklist.contains(l))
+        .cloned()
+        .collect();
+
+    match filtered.len() {
+        // Case A: If all labels were status labels, fallback to the first original label
+        0 => all_labels[0].to_string(),
+        // Case B: If only one label remains, that's our target destination
+        1 => filtered[0].to_string(),
+        // Case C: Multiple labels remain (e.g., ["Inbox", "medium"])
+        _ => {
+            // Prioritize custom business labels by excluding generic locations like "Inbox" or "Sent"
+            let business_label = filtered.iter().find(|&&l| l != "Inbox" && l != "Sent");
+
+            match business_label {
+                // Return the first non-generic label found
+                Some(label) => label.to_string(),
+                // If only generic labels remain (e.g., ["Sent", "Inbox"]), pick the first available
+                None => filtered[0].to_string(),
+            }
+        }
+    }
+}