feat: Case-insensitive search for non-ASCII messages (#5052)

SQLite search with `LIKE` is case-insensitive only for ASCII chars. To make it case-insensitive for
all messages, create a new column `msgs.txt_normalized` defaulting to `NULL` (so we do not bump up
the database size in a migration) and storing lowercased/normalized text there when the row is
created/updated. When doing a search, search over `IFNULL(txt_normalized, txt)`.

src/message.rs

@@ -113,7 +113,7 @@ impl MsgId {
                 r#"
 UPDATE msgs 
 SET 
-  chat_id=?, txt='', 
+  chat_id=?, txt='', txt_normalized=NULL, 
   subject='', txt_raw='', 
   mime_headers='', 
   from_id=0, to_id=0, 
@@ -2072,6 +2072,15 @@ impl Viewtype {
     }
 }
 
+/// Returns text for storing in the `msgs.txt_normalized` column (to make case-insensitive search
+/// possible for non-ASCII messages).
+pub(crate) fn normalize_text(text: &str) -> Option<String> {
+    if text.is_ascii() {
+        return None;
+    };
+    Some(text.to_lowercase()).filter(|t| t != text)
+}
+
 #[cfg(test)]
 mod tests {
     use num_traits::FromPrimitive;