Add history pruning with configurable max entries and max days

- Add history_max_entries (default 50) and history_max_days (default 30)
  to AppConfig
- Add prune() method to HistoryStore that removes old entries by age
  and count limits
- Call prune after each history entry is added in the GUI
- Preserve history settings through settings dialog save/load cycle

pixstrip-core/src/config.rs

@@ -17,6 +17,8 @@ pub struct AppConfig {
     pub high_contrast: bool,
     pub large_text: bool,
     pub reduced_motion: bool,
+    pub history_max_entries: usize,
+    pub history_max_days: u32,
 }
 
 impl Default for AppConfig {
@@ -36,6 +38,8 @@ impl Default for AppConfig {
             high_contrast: false,
             large_text: false,
             reduced_motion: false,
+            history_max_entries: 50,
+            history_max_days: 30,
         }
     }
 }