feature: PDF and PDF thumbnails and refresh UI

- Implement PDF and PDF thumbnail generation with incremental loading
- Add UI refresh mechanism (tick counter + RefreshView message)
- Improve fl! macro with named parameters
- Clean up code organization (mod.rs: wiring, model.rs: state only)

src/constant.rs

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/constant.rs
+//
+// Application constants that should not be changed by the user.
+
+/// Rotation step in degrees (90 = quarter turn).
+pub const ROTATION_STEP: i16 = 90;
+
+/// Full rotation in degrees (for modulo calculation in angle normalization).
+pub const FULL_ROTATION: i16 = 360;
+
+/// Minutes per degree (GPS coordinate conversion: DMS to decimal degrees).
+pub const MINUTES_PER_DEGREE: f64 = 60.0;
+
+/// Seconds per degree (GPS coordinate conversion: DMS to decimal degrees).
+pub const SECONDS_PER_DEGREE: f64 = 3600.0;
+
+/// Minimum pixmap size for SVG rendering (prevents 0x0 images).
+pub const MIN_PIXMAP_SIZE: u32 = 1;
+
+/// Tolerance for scale comparisons (float precision in zoom synchronization).
+pub const SCALE_EPSILON: f32 = 0.0001;
+
+/// Tolerance for offset comparisons (float precision in pan synchronization).
+pub const OFFSET_EPSILON: f32 = 0.01;
+
+/// Maximum thumbnail width in pixels (nav bar page thumbnails).
+pub const THUMBNAIL_MAX_WIDTH: f32 = 100.0;
+
+/// Thumbnail cache directory name.
+pub const CACHE_DIR: &str = "noctua";
+
+/// Thumbnail file extension.
+pub const THUMBNAIL_EXT: &str = "png";
+
+/// Default render scale for PDF pages.
+pub const PDF_RENDER_SCALE: f64 = 2.0;
+
+/// Thumbnail render scale (smaller for quick rendering).
+pub const PDF_THUMBNAIL_SCALE: f64 = 0.25;