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/document/file.rs

@@ -79,11 +79,10 @@ pub fn open_single_file(model: &mut AppModel, path: &Path) {
     load_document_into_model(model, path);
 
     // Refresh folder listing based on parent directory.
-    if model.document.is_some() {
-        if let Some(parent) = path.parent() {
+    if model.document.is_some()
+        && let Some(parent) = path.parent() {
             refresh_folder_entries(model, parent, path);
         }
-    }
 }
 
 /// Load a document into the model, resetting view state.