fix(goals/show): add Required line to projection legend

The dashed neutral line for "the rate needed to hit the target by
the target date" was rendered on the chart but absent from the
legend. Adds a third legend chip ("Required") that renders only
when the line itself renders — i.e. when target_date is set,
monthly_target_amount is positive, and there's still ground to
cover (remaining_amount > 0). Stroke matches the JS:
`text-secondary` currentColor, dasharray 2/4, 0.5 opacity.

app/views/goals/show.html.erb

@@ -243,6 +243,12 @@
                 <svg width="18" height="6"><line x1="0" y1="3" x2="18" y2="3" stroke="<%= projection_color %>" stroke-width="2" stroke-dasharray="3 3" /></svg>
                 <%= t(".projection.legend_projection") %>
               </span>
+              <% if @goal.monthly_target_amount.to_d.positive? && @goal.remaining_amount.to_d.positive? %>
+                <span class="inline-flex items-center gap-1.5">
+                  <svg width="18" height="6" class="text-secondary"><line x1="0" y1="3" x2="18" y2="3" stroke="currentColor" stroke-width="2" stroke-dasharray="2 4" opacity="0.5" /></svg>
+                  <%= t(".projection.legend_required") %>
+                </span>
+              <% end %>
             <% end %>
           </div>
         </div>

config/locales/views/goals/en.yml

@@ -127,6 +127,7 @@ en:
         heading: Projection
         legend_saved: Saved
         legend_projection: Projection
+        legend_required: Required
         reached: Goal reached. Nice work.
         no_target_date: No target date set. Set one to project a finish line.
         no_pace: No inflow yet. Make a transfer or update the balance to start a projection.