refactor: improve code quality and consistency

Comprehensive code audit addressing naming consistency, dead code removal,
and documentation improvements across the entire codebase.

- Rename panel_pages.rs → pages_panel.rs for consistent naming
- Unify view functions: all panels now use view() convention
- Rename header_start/end() → start/end() for brevity
- Update all imports and references

- Remove Rotation::from_degrees() (only 4 fixed states needed)
- Remove TransformState::identity() and is_identity() (Default suffices)
- Remove duplicate convenience methods from RasterDocument, VectorDocument,
  PortableDocument (rotate_cw/ccw, flip_horizontal/vertical)
- Remove unused imports and constants (ROTATION_STEP, FULL_ROTATION)

- Rename constants: PDF_RENDER_SCALE → PDF_RENDER_QUALITY,
  PDF_THUMBNAIL_SCALE → PDF_THUMBNAIL_SIZE
- Add comprehensive trait documentation explaining type erasure pattern
- Document why large enum variants are acceptable
- Add #[allow(dead_code)] with explanations for trait API types
- Improve all constant and config comments

- Collapse nested if statements using Rust 2024 let-chains
- Replace single-arm match with if-let
- Introduce StateChangeCallback<Message> type alias
- Apply clippy auto-fixes for better code style

src/app/mod.rs

@@ -166,11 +166,11 @@ impl cosmic::Application for Noctua {
     }
 
     fn header_start(&self) -> Vec<Element<'_, Self::Message>> {
-        view::header::header_start(&self.model)
+        view::header::start(&self.model)
     }
 
     fn header_end(&self) -> Vec<Element<'_, Self::Message>> {
-        view::header::header_end(&self.model)
+        view::header::end(&self.model)
     }
 
     fn view(&self) -> Element<'_, Self::Message> {
@@ -182,7 +182,7 @@ impl cosmic::Application for Noctua {
             return None;
         }
         Some(context_drawer::context_drawer(
-            view::panels::properties_panel(&self.model),
+            view::panels::view(&self.model),
             AppMessage::ToggleContextPage(ContextPage::Properties),
         ))
     }
@@ -307,7 +307,7 @@ fn thumbnail_refresh_subscription(app: &Noctua) -> Subscription<AppMessage> {
         .model
         .document
         .as_ref()
-        .map_or(false, |doc| doc.is_multi_page() && !doc.thumbnails_ready());
+        .is_some_and(|doc| doc.is_multi_page() && !doc.thumbnails_ready());
 
     if needs_refresh {
         time::every(Duration::from_millis(100)).map(|_| AppMessage::RefreshView)