refactor: centralize file handling, fix zoom display and cleanup

File handling (document/file.rs):
- move file operations from app/mod.rs to document/file.rs
- add open_file_dialog() for native file picker
- add collect_directory_siblings() for navigation context
- add open_document_from_path() as main entry point

Zoom/View (panels.rs, canvas.rs, model.rs):
- fix zoom display using ViewMode enum
- ViewMode::Fit shows Fit, ActualSize shows 100%, Custom shows percentage

Model/Update cleanup:
- adjust model.rs for new file handling
- update.rs: use centralized file functions
- document/mod.rs: re-exports for file module

i18n:
BB
ctua.ftl with new/changed strings"
A
- update noctua.ftl with new/changed strings"

src/app/view/canvas.rs

@@ -7,9 +7,9 @@ use cosmic::iced::{Alignment, Length};
 use cosmic::widget::{container, image, text, Column, Row};
 use cosmic::Element;
 
-use crate::fl;
 use crate::app::model::ViewMode;
 use crate::app::{AppMessage, AppModel};
+use crate::fl;
 
 /// Render the center canvas area with the current document.
 pub fn view(model: &AppModel) -> Element<'_, AppMessage> {
@@ -30,11 +30,11 @@ pub fn view(model: &AppModel) -> Element<'_, AppMessage> {
                     .width(Length::Fixed(native_w as f32))
                     .height(Length::Fixed(native_h as f32))
             }
-            ViewMode::Custom(_) => {
+            ViewMode::Custom(zoom) => {
                 // Custom zoom factor applied to native size.
                 let (native_w, native_h) = doc.dimensions();
-                let scaled_w = (native_w as f32 * model.zoom).round();
-                let scaled_h = (native_h as f32 * model.zoom).round();
+                let scaled_w = (native_w as f32 * zoom).round();
+                let scaled_h = (native_h as f32 * zoom).round();
                 image::Image::new(handle)
                     .width(Length::Fixed(scaled_w))
                     .height(Length::Fixed(scaled_h))