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/storage.rs

@@ -272,6 +272,32 @@ impl HistoryStore {
         self.write_all(&entries)
     }
 
+    pub fn prune(&self, max_entries: usize, max_days: u32) -> Result<()> {
+        let mut entries = self.list()?;
+        if entries.is_empty() {
+            return Ok(());
+        }
+
+        let now_secs = std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .unwrap_or_default()
+            .as_secs();
+        let cutoff_secs = now_secs.saturating_sub(max_days as u64 * 86400);
+
+        // Remove entries older than max_days
+        entries.retain(|e| {
+            e.timestamp.parse::<u64>().unwrap_or(0) >= cutoff_secs
+        });
+
+        // Trim to max_entries (keep the most recent)
+        if entries.len() > max_entries {
+            let start = entries.len() - max_entries;
+            entries = entries.split_off(start);
+        }
+
+        self.write_all(&entries)
+    }
+
     pub fn list(&self) -> Result<Vec<HistoryEntry>> {
         if !self.history_path.exists() {
             return Ok(Vec::new());