refactor: apply clippy auto-fixes (162 warnings resolved)

- Fix needless borrows and references
- Collapse nested if statements
- Replace manual strip_prefix with str::strip_prefix()
- Remove redundant closures in map/unwrap_or_else
- Use Iterator::next_back() instead of rev().next()
- Simplify map_or patterns
- Use std::io::Error::other() instead of new(ErrorKind::Other, ..)
- Use div_ceil() instead of manual ceiling division
- Consolidate format! string arguments
- Various other idiomatic Rust improvements

39 files changed, 220 insertions(+), 320 deletions(-)

src/interfaces/api/handlers/caldav_handler.rs

@@ -320,7 +320,7 @@ async fn handle_mkcalendar(
         .map_err(|e| AppError::bad_request(format!("Failed to read request body: {}", e)))?;
     
     let (name, description, color) = if body_bytes.is_empty() {
-        let name = path.split('/').last().unwrap_or("New Calendar").to_string();
+        let name = path.split('/').next_back().unwrap_or("New Calendar").to_string();
         (name, None, None)
     } else {
         CalDavAdapter::parse_mkcalendar(body_bytes.reader())