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/config.rs

@@ -10,19 +10,19 @@ use std::path::PathBuf;
 #[derive(Debug, Clone, CosmicConfigEntry, PartialEq)]
 #[version = 1]
 pub struct AppConfig {
-    /// Optional default directory to open images from.
+    /// Default directory to open when browsing for documents.
     pub default_image_dir: Option<PathBuf>,
-    /// Whether the nav bar (left panel) is visible.
+    /// Show page navigation panel (left sidebar for multi-page documents).
     pub nav_bar_visible: bool,
-    /// Whether the context drawer (right panel) is visible.
+    /// Show properties panel (right sidebar with metadata).
     pub context_drawer_visible: bool,
-    /// Scale step factor for keyboard zoom (e.g., 1.1 = 10% per step).
+    /// Zoom step multiplier for keyboard shortcuts (1.1 = 10% increase per step).
     pub scale_step: f32,
-    /// Pan step size in pixels per key press.
+    /// Pan distance in pixels per arrow key press.
     pub pan_step: f32,
-    /// Minimum zoom scale (e.g., 0.1 = 10%).
+    /// Minimum zoom level (0.1 = 10% of original size).
     pub min_scale: f32,
-    /// Maximum zoom scale (e.g., 20.0 = 2000%).
+    /// Maximum zoom level (8.0 = 800% of original size).
     pub max_scale: f32,
 }