fix(03): WR-04 add color field error display to create form template

- TabloCreateErrors: add Color field for server-side hex validation error
- TabloCreateFormFragment: render FieldError for color field and update
  placeholder to hex-only hint (#6366f1) matching the validation constraint

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

backend/templates/tablos.templ

@@ -139,8 +139,9 @@ templ TabloCreateFormFragment(form TabloCreateForm, errs TabloCreateErrors, csrf
 				name="color"
 				value={ form.Color }
 				class="mt-1 block w-full rounded border border-slate-300 px-3 py-2 text-sm placeholder-slate-400 focus:border-slate-500 focus:outline-none"
-				placeholder="#6366f1 or indigo"
+				placeholder="#6366f1"
 			/>
+			@FieldError(errs.Color)
 		</div>
 		<div class="flex items-center gap-3">
 			@ui.Button(ui.ButtonProps{

backend/templates/tablos_forms.go

@@ -12,6 +12,7 @@ type TabloCreateForm struct {
 // create form. A field with an empty string means "no error for this field".
 type TabloCreateErrors struct {
 	Title   string
+	Color   string
 	General string
 }