fix: Model::add_formula auto-adds target item; fix formula panel display

Model::add_formula now ensures the formula target exists as an item in
the target category (same fix as AddFormula effect, but at the model
level — covers file loading and headless paths).

Formula panel now shows raw formula text instead of debug-formatted
string with redundant target name and quotes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ui/formula_panel.rs

@@ -50,7 +50,7 @@ impl PanelContent for FormulaContent<'_> {
         } else {
             Style::default().fg(Color::Green)
         };
-        let text = format!("  {} = {:?}", formula.target, formula.raw);
+        let text = format!("  {}", formula.raw);
         let truncated = if text.len() > inner.width as usize {
             format!("{}…", &text[..inner.width as usize - 1])
         } else {