Add marking mode; rename resize and zoom methods

Introduce a marking mode that lets users toggle cells with Space and
clear
them with Backspace. BoardState now stores `marking_cells` and a list of
`marked_cells` and provides methods to start, clear, and update marks.
CellWidget tracks a `marked` flag and renders marked cells in
LightMagenta.

Keybindings are updated: Space now reads “Marking cells”, Backspace
reads
“Cancel marking”, and the original Enter binding is commented out.

The former `resize_change` and `zoom_change` functions are renamed to
`change_resize` and `change_zoom`; all call sites are updated
accordingly.

src/app/widgets/cell.rs

@@ -27,6 +27,7 @@ pub struct CellWidget {
     selected: bool,
     zoom_level: ZoomLevel,
     tag: CellTag,
+    marked: bool,
 }
 
 impl CellWidget {
@@ -43,6 +44,7 @@ impl CellWidget {
             selected,
             zoom_level,
             tag,
+            marked: false,
         }
     }
 
@@ -58,6 +60,14 @@ impl CellWidget {
         self.tag = tag;
     }
 
+    pub fn get_marked(&self) -> bool {
+        self.marked
+    }
+
+    pub fn set_marked(&mut self, marked: bool) {
+        self.marked = marked
+    }
+
     fn col_to_letters(&self) -> String {
         let mut col: usize = self.col + 1;
         let mut letters: Vec<char> = Vec::new();
@@ -80,6 +90,7 @@ impl CellWidget {
 
     fn fg_color(&self) -> Color {
         match self.tag {
+            _ if self.marked => Color::LightMagenta,
             _ if self.selected => Color::LightYellow,
             CellTag::Base(Players::Player) if !self.selected => Color::LightBlue,
             CellTag::Base(Players::Enemy) if !self.selected => Color::LightRed,