Encode folder names as modified UTF-7 over IMAP

Add an imap::utf7 module (RFC 3501 §5.1.3) and apply it at the protocol
boundary: encode mailbox names in LIST, decode them in SELECT/STATUS.
Folder names internally stay UTF-8; only the IMAP wire form is UTF-7.

Also fix STATUS argument parsing to handle quoted mailbox names with
spaces (it split on the first space, truncating names like
"Not Important" or nested paths).

Live-tested: "Café" lists as "Caf&AOk-", a nested child lists as
"Caf&AOk-/Test dossier avec espace", and SELECT/STATUS resolve both.

crates/bridge/src/imap/mod.rs

@@ -1,4 +1,5 @@
 mod session;
+mod utf7;
 
 use std::sync::Arc;
 use log::{info, error, debug};

crates/bridge/src/imap/session.rs

@@ -220,7 +220,8 @@ impl ImapSession {
             let flags = folder.special_use.as_deref().unwrap_or("");
             responses.push(format!(
                 "* LIST ({}) \"/\" \"{}\"\r\n",
-                flags, folder.imap_path
+                flags,
+                super::utf7::encode(&folder.imap_path)
             ));
         }
         responses.push(format!("{} OK LIST completed\r\n", tag));
@@ -233,8 +234,9 @@ impl ImapSession {
             return vec![format!("{} NO Not authenticated\r\n", tag)];
         }
 
-        let folder_name = args.trim().trim_matches('"');
-        let folder = match self.store.folder_by_imap_path(folder_name).await {
+        let raw_name = args.trim().trim_matches('"');
+        let folder_name = super::utf7::decode(raw_name).unwrap_or_else(|| raw_name.to_string());
+        let folder = match self.store.folder_by_imap_path(&folder_name).await {
             Some(f) => f,
             None => {
                 return vec![format!("{} NO [NONEXISTENT] Mailbox does not exist\r\n", tag)];
@@ -283,8 +285,9 @@ impl ImapSession {
         if self.state == State::NotAuthenticated {
             return vec![format!("{} NO Not authenticated\r\n", tag)];
         }
-        let folder_name = args.split_whitespace().next().unwrap_or("").trim_matches('"');
-        let stored = match self.store.folder_by_imap_path(folder_name).await {
+        let (raw_name, _) = parse_imap_token(args.trim());
+        let folder_name = super::utf7::decode(&raw_name).unwrap_or_else(|| raw_name.clone());
+        let stored = match self.store.folder_by_imap_path(&folder_name).await {
             Some(folder) => self.store.get_folder(&folder.id).await,
             None => Vec::new(),
         };
@@ -293,7 +296,7 @@ impl ImapSession {
         vec![
             format!(
                 "* STATUS \"{}\" (MESSAGES {} UNSEEN {} RECENT 0 UIDNEXT {} UIDVALIDITY 1)\r\n",
-                folder_name, count, unseen, self.uid_next
+                raw_name, count, unseen, self.uid_next
             ),
             format!("{} OK STATUS completed\r\n", tag),
         ]

crates/bridge/src/imap/utf7.rs

@@ -0,0 +1,139 @@
+//! Modified UTF-7 for IMAP mailbox names (RFC 3501 §5.1.3).
+//!
+//! Printable ASCII (0x20–0x7E) represents itself, except `&` which is encoded
+//! as `&-`. Any other run of characters is encoded as `&<b64>-` where `<b64>`
+//! is the modified BASE64 (alphabet `+,` instead of `+/`, no padding) of the
+//! run encoded as UTF-16BE.
+
+use base64::alphabet::Alphabet;
+use base64::engine::general_purpose::{GeneralPurpose, GeneralPurposeConfig};
+use base64::engine::DecodePaddingMode;
+use base64::Engine;
+use std::sync::LazyLock;
+
+/// Modified BASE64 alphabet: standard, but `/` becomes `,`.
+static MODIFIED_B64: LazyLock<GeneralPurpose> = LazyLock::new(|| {
+    let alphabet =
+        Alphabet::new("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,")
+            .expect("valid alphabet");
+    let config = GeneralPurposeConfig::new()
+        .with_encode_padding(false)
+        .with_decode_padding_mode(DecodePaddingMode::RequireNone);
+    GeneralPurpose::new(&alphabet, config)
+});
+
+fn is_direct(c: char) -> bool {
+    // Printable ASCII except `&`.
+    ('\u{20}'..='\u{7e}').contains(&c) && c != '&'
+}
+
+/// Encode a UTF-8 mailbox name into modified UTF-7.
+pub fn encode(input: &str) -> String {
+    let mut out = String::with_capacity(input.len());
+    let mut shifted: Vec<u16> = Vec::new();
+
+    let flush = |shifted: &mut Vec<u16>, out: &mut String| {
+        if shifted.is_empty() {
+            return;
+        }
+        let mut bytes = Vec::with_capacity(shifted.len() * 2);
+        for unit in shifted.drain(..) {
+            bytes.extend_from_slice(&unit.to_be_bytes());
+        }
+        out.push('&');
+        out.push_str(&MODIFIED_B64.encode(&bytes));
+        out.push('-');
+    };
+
+    for c in input.chars() {
+        if c == '&' {
+            flush(&mut shifted, &mut out);
+            out.push_str("&-");
+        } else if is_direct(c) {
+            flush(&mut shifted, &mut out);
+            out.push(c);
+        } else {
+            let mut buf = [0u16; 2];
+            shifted.extend_from_slice(c.encode_utf16(&mut buf));
+        }
+    }
+    flush(&mut shifted, &mut out);
+    out
+}
+
+/// Decode a modified UTF-7 mailbox name back to UTF-8. Returns `None` if the
+/// input is not valid modified UTF-7.
+pub fn decode(input: &str) -> Option<String> {
+    let mut out = String::with_capacity(input.len());
+    let bytes = input.as_bytes();
+    let mut i = 0;
+
+    while i < bytes.len() {
+        let b = bytes[i];
+        if b == b'&' {
+            // Find the closing '-'.
+            let end = bytes[i + 1..].iter().position(|&x| x == b'-')? + i + 1;
+            let chunk = &input[i + 1..end];
+            if chunk.is_empty() {
+                // "&-" => literal '&'
+                out.push('&');
+            } else {
+                let decoded = MODIFIED_B64.decode(chunk.as_bytes()).ok()?;
+                if decoded.len() % 2 != 0 {
+                    return None;
+                }
+                let units: Vec<u16> = decoded
+                    .chunks_exact(2)
+                    .map(|c| u16::from_be_bytes([c[0], c[1]]))
+                    .collect();
+                out.push_str(&String::from_utf16(&units).ok()?);
+            }
+            i = end + 1;
+        } else {
+            // Direct ASCII byte.
+            out.push(b as char);
+            i += 1;
+        }
+    }
+    Some(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn roundtrip(plain: &str, wire: &str) {
+        assert_eq!(encode(plain), wire, "encode {plain:?}");
+        assert_eq!(decode(wire).as_deref(), Some(plain), "decode {wire:?}");
+    }
+
+    #[test]
+    fn ascii_is_identity() {
+        roundtrip("INBOX", "INBOX");
+        roundtrip("Work/Projects", "Work/Projects");
+        roundtrip("Not Important", "Not Important");
+    }
+
+    #[test]
+    fn ampersand_is_escaped() {
+        roundtrip("R&D", "R&-D");
+        roundtrip("&", "&-");
+    }
+
+    #[test]
+    fn non_ascii_is_shifted() {
+        // Examples from RFC 3501.
+        roundtrip("Café", "Caf&AOk-");
+        roundtrip("~peter/mail/台北/日本語", "~peter/mail/&U,BTFw-/&ZeVnLIqe-");
+    }
+
+    #[test]
+    fn mixed_runs() {
+        roundtrip("Dossier éàü test", "Dossier &AOkA4AD8- test");
+    }
+
+    #[test]
+    fn decode_rejects_unterminated_shift() {
+        assert_eq!(decode("&AOk"), None);
+    }
+}