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/document/mod.rs

@@ -14,7 +14,7 @@ pub mod vector;
 use cosmic::iced::widget::image as iced_image;
 use cosmic::iced_renderer::graphics::image::image_rs::ImageFormat as CosmicImageFormat;
 use std::fmt;
-use std::path::{Path, PathBuf};
+use std::path::Path;
 
 use self::portable::PortableDocument;
 use self::raster::RasterDocument;
@@ -89,15 +89,6 @@ impl DocumentContent {
         }
     }
 
-    /// Returns the underlying filesystem path of this document, if any.
-    pub fn path(&self) -> Option<&PathBuf> {
-        match self {
-            DocumentContent::Raster(doc) => doc.path.as_ref(),
-            DocumentContent::Vector(doc) => Some(&doc.path),
-            DocumentContent::Portable(doc) => Some(&doc.path),
-        }
-    }
-
     /// Returns the native dimensions (width, height) of the document in pixels.
     ///
     /// For raster images this is the actual pixel size.