fix: resolve clippy warnings

Reduce clippy warnings from54 to16 by fixing mechanical issues:

- collapsible_if: collapse nested if-let chains with let-chains
- clone_on_copy: remove unnecessary .clone() on Copy types
- manual_clamp: replace .max().min() with .clamp()
- unnecessary_cast: remove redundant type casts
- collapsible_else_if: flatten else-if chains
- contains_vs_iter_any: replace .iter().any() with .contains()
- unnecessary_closure: replace .or_else(|| x) with .or(x)
- useless_conversion: remove redundant .into() calls
- is_none_or: replace .map_or(true, ...) with .is_none_or(...)
- while_let_loop: convert loop with if-let-break to while-let

Remaining16 warnings are design-level issues (too_many_arguments,
await_holding_lock, type_complexity, new_ret_no_self) that require
architectural changes to fix.

src/config/types.rs

@@ -677,9 +677,10 @@ pub struct ListenerConfig {
 /// - `show_link = "*"`          — show links for all users
 /// - `show_link = ["a", "b"]`   — show links for specific users
 /// - omitted                    — show no links (default)
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Default)]
 pub enum ShowLink {
     /// Don't show any links (default when omitted).
+    #[default]
     None,
     /// Show links for all configured users.
     All,
@@ -687,12 +688,6 @@ pub enum ShowLink {
     Specific(Vec<String>),
 }
 
-impl Default for ShowLink {
-    fn default() -> Self {
-        ShowLink::None
-    }
-}
-
 impl ShowLink {
     /// Returns true if no links should be shown.
     pub fn is_empty(&self) -> bool {