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/update.rs

@@ -39,17 +39,16 @@ pub fn update(model: &mut AppModel, msg: &AppMessage, config: &AppConfig) -> Upd
         }
 
         AppMessage::GotoPage(page) => {
-            if let Some(doc) = &mut model.document {
-                if let Err(e) = doc.goto_page(*page) {
+            if let Some(doc) = &mut model.document
+                && let Err(e) = doc.go_to_page(*page) {
                     log::error!("Failed to navigate to page {}: {}", page, e);
                 }
-            }
         }
 
         // ---- Thumbnail generation -------------------------------------------------
         AppMessage::GenerateThumbnailPage(page) => {
-            if let Some(doc) = &mut model.document {
-                if let Some(next_page) = doc.generate_thumbnail_page(*page) {
+            if let Some(doc) = &mut model.document
+                && let Some(next_page) = doc.generate_thumbnail_page(*page) {
                     return UpdateResult::Task(Task::batch([
                         Task::future(async move {
                             Action::App(AppMessage::GenerateThumbnailPage(next_page))
@@ -57,7 +56,6 @@ pub fn update(model: &mut AppModel, msg: &AppMessage, config: &AppConfig) -> Upd
                         Task::done(Action::App(AppMessage::RefreshView)),
                     ]));
                 }
-            }
         }
 
         AppMessage::RefreshView => {