refactor: break down handle_input into smaller methods in src/gui/input.rs (#67)

Extract sections from the long `handle_input` function into smaller,
context-specific helper methods such as `handle_hotkey_assignment`,
`handle_toggles`, `handle_playback_and_focus`, `handle_file_playback`,
`handle_navigation`, and `handle_hotkey_triggers`. This significantly
improves the maintainability and readability of `src/gui/input.rs`
while preserving original functionality.

In addition, ran `cargo clippy --fix` on the project to resolve a few
other minor health issues, like collapsing nested `if` statements and
reducing unnecessary allocations.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

src/types/commands.rs

@@ -673,7 +673,7 @@ impl Executable for PlayHotkeyCommand {
         if let Some(cmd) = parse_command(&action) {
             cmd.execute().await
         } else {
-            Response::new(false, "Unknown command in hotkey slot".to_string())
+            Response::new(false, "Unknown command in hotkey slot")
         }
     }
 }