Persist advanced options expand/collapse state per section

Each ExpanderRow now remembers its expanded state between sessions
using a per-section key stored in SessionState. Replaces the global
detailed_mode toggle with granular per-section persistence.

pixstrip-core/src/storage.rs

@@ -189,6 +189,7 @@ pub struct SessionState {
     pub watermark_enabled: Option<bool>,
     pub rename_enabled: Option<bool>,
     pub last_seen_version: Option<String>,
+    pub expanded_sections: std::collections::HashMap<String, bool>,
 }
 
 pub struct SessionStore {