perf(gui): eliminate allocations during cache file read using BufReader (#192)

* perf: eliminate allocations during cache file read using BufReader

Replaced `fs::read_to_string` combined with `serde_json::from_str` with `std::fs::File::open` and `serde_json::from_reader(std::io::BufReader::new(file))`. This avoids loading the entire JSON file into a string allocation before parsing it, improving memory usage.

Co-authored-by: arabianq <55220741+arabianq@users.noreply.github.com>

* small reformatting

---------

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

pwsp-gui/src/gui/mod.rs

@@ -23,6 +23,7 @@ use std::{
     collections::{HashMap, HashSet},
     fs,
     hash::{DefaultHasher, Hash, Hasher},
+    io::BufReader,
     path::{Path, PathBuf},
     sync::{Arc, Mutex},
     thread,
@@ -175,11 +176,9 @@ impl SoundpadGui {
 
                 // 1. Try to load from disk cache if we don't have it in memory yet
                 if !is_cached
-                    && let Ok(data) = fs::read_to_string(&cache_file)
-                    && let Ok((all_files, dir_updates)) = serde_json::from_str::<(
-                        Vec<PathBuf>,
-                        HashMap<PathBuf, Vec<PathBuf>>,
-                    )>(&data)
+                    && let Ok(file) = fs::File::open(&cache_file)
+                    && let Ok((all_files, dir_updates)) =
+                        serde_json::from_reader(BufReader::new(file))
                 {
                     finished_scans.lock().unwrap().push((
                         path_clone.clone(),