refactor(goals/show): move projection_summary + catch_up_delta to model

The show template carried a 17-line `if/elsif` chain computing
`projection_summary` inline, plus a `Money.new([…, 0].max, …)`
expression building the catch-up delta on the fly. CLAUDE.md's
"skinny controllers, fat models" convention pushes both onto Goal.

- `Goal#projection_summary`: returns the localized,
  `html_safe`-aware string for the chart subtitle and the chart's
  `aria-description`. Memoized so the two callsites in show.html.erb
  share one computation.
- `Goal#catch_up_delta_money`: clamped-at-zero monthly delta between
  pace and the required monthly target. Used by the catch-up
  callout body. Previously the view computed
  `Money.new([req - pace, 0].max, currency)` — same math, but
  duplicated inline.

show.html.erb drops both blocks and reads `@goal.projection_summary`
/ `@goal.catch_up_delta_money` directly.

Also: V15 — the celebration card used `bg-green-500/10` directly.
Swap to `bg-success/10` (DS semantic token, same Tailwind-4 alpha
syntax DS::Alert already uses) so the celebration palette tracks
the rest of the success surface.

app/models/goal.rb

@@ -249,6 +249,44 @@ class Goal < ApplicationRecord
     any_connected_account? ? "goals.show.pledge_just_transferred" : "goals.show.pledge_just_saved"
   end
 
+  # Single source of truth for the projection-chart subtitle / chart-aria
+  # description. Used to live inline in show.html.erb as a 17-line if/elsif
+  # chain. Returns an `html_safe` string when it picks the `_html` variant.
+  def projection_summary
+    return @projection_summary if defined?(@projection_summary)
+
+    @projection_summary =
+      if completed? || progress_percent >= 100
+        I18n.t("goals.show.projection.reached")
+      elsif target_date.nil?
+        I18n.t("goals.show.projection.no_target_date")
+      elsif monthly_target_amount && pace.to_d < monthly_target_amount.to_d
+        I18n.t(
+          "goals.show.projection.behind",
+          current: Money.new(pace, currency).format,
+          required: Money.new(monthly_target_amount, currency).format
+        )
+      elsif pace.positive?
+        months = (remaining_amount.to_d / pace.to_d).ceil
+        I18n.t(
+          "goals.show.projection.on_track_html",
+          date: (Date.current >> months.to_i).strftime("%b %Y")
+        )
+      else
+        I18n.t("goals.show.projection.no_pace")
+      end
+  end
+
+  # Monthly extra needed beyond the current pace to hit the target on time.
+  # Clamps at zero — never asks the user to "make up" a deficit they're
+  # already ahead of.
+  def catch_up_delta_money
+    return Money.new(0, currency) if monthly_target_amount.nil?
+
+    delta = [ monthly_target_amount.to_d - pace.to_d, 0 ].max
+    Money.new(delta, currency)
+  end
+
   private
     def balance_series_values
       return [] if linked_accounts.empty?

app/views/goals/show.html.erb

@@ -136,14 +136,12 @@
     <% end %>
   <% elsif @goal.status == :behind && @goal.monthly_target_amount %>
     <% catch_up_money = Money.new(@goal.monthly_target_amount, @goal.currency) %>
-    <% catch_up_pace_money = @goal.pace_money %>
-    <% catch_up_delta_money = Money.new([ @goal.monthly_target_amount.to_d - @goal.pace.to_d, 0 ].max, @goal.currency) %>
     <%= render DS::Alert.new(variant: "warning", title: t("goals.show.catch_up.title", amount: catch_up_money.format)) do %>
       <p class="text-secondary">
         <% if @goal.target_date %>
-          <%= t("goals.show.catch_up.body_with_date", avg: catch_up_pace_money.format, delta: catch_up_delta_money.format, date: I18n.l(@goal.target_date, format: :long)) %>
+          <%= t("goals.show.catch_up.body_with_date", avg: @goal.pace_money.format, delta: @goal.catch_up_delta_money.format, date: I18n.l(@goal.target_date, format: :long)) %>
         <% else %>
-          <%= t("goals.show.catch_up.body", avg: catch_up_pace_money.format, delta: catch_up_delta_money.format) %>
+          <%= t("goals.show.catch_up.body", avg: @goal.pace_money.format, delta: @goal.catch_up_delta_money.format) %>
         <% end %>
       </p>
       <div class="mt-2 flex items-center gap-3 flex-wrap">
@@ -176,26 +174,6 @@
       </p>
     </div>
 
-    <%
-      pace = @goal.pace
-      required = @goal.monthly_target_amount
-      projection_summary = if @goal.completed? || @goal.progress_percent >= 100
-        t("goals.show.projection.reached")
-      elsif @goal.target_date.nil?
-        t("goals.show.projection.no_target_date")
-      elsif required && pace.to_d < required.to_d
-        t("goals.show.projection.behind",
-          current: Money.new(pace, @goal.currency).format,
-          required: Money.new(required, @goal.currency).format)
-      elsif pace.positive?
-        months = (@goal.remaining_amount.to_d / pace.to_d).ceil
-        t("goals.show.projection.on_track_html",
-          date: (Date.current >> months.to_i).strftime("%b %Y"))
-      else
-        t("goals.show.projection.no_pace")
-      end
-    %>
-
     <% if @goal.archived? || @goal.paused? %>
       <div class="bg-container rounded-xl shadow-border-xs p-5 flex flex-col items-center justify-center text-center">
         <div class="w-16 h-16 rounded-full bg-surface-inset inline-flex items-center justify-center text-secondary mb-3">
@@ -210,7 +188,7 @@
       </div>
     <% elsif @goal.completed? || @goal.status == :reached %>
       <div class="bg-container rounded-xl shadow-border-xs p-5 flex flex-col items-center justify-center text-center">
-        <div class="w-16 h-16 rounded-full bg-green-500/10 inline-flex items-center justify-center text-success mb-3">
+        <div class="w-16 h-16 rounded-full bg-success/10 inline-flex items-center justify-center text-success mb-3">
           <%= icon("party-popper", size: "2xl", color: "success") %>
         </div>
         <h2 class="text-lg font-semibold text-primary"><%= t(".celebration.heading") %></h2>
@@ -252,7 +230,7 @@
         <div class="flex items-start justify-between mb-2 gap-3">
           <div class="min-w-0">
             <h2 class="text-sm font-medium text-primary"><%= t(".projection.heading") %></h2>
-            <p class="text-xs text-secondary mt-0.5"><%= projection_summary %></p>
+            <p class="text-xs text-secondary mt-0.5"><%= @goal.projection_summary %></p>
           </div>
           <% projection_color = @goal.status == :on_track ? "var(--color-green-600)" : "var(--color-yellow-600)" %>
           <div class="flex items-center gap-3 text-[11px] text-secondary shrink-0">
@@ -272,7 +250,7 @@
              data-controller="goal-projection-chart"
              data-goal-projection-chart-data-value="<%= @goal.projection_payload.to_json %>"
              data-goal-projection-chart-aria-label-value="<%= t("goals.show.projection.aria_label", name: @goal.name) %>"
-             data-goal-projection-chart-aria-description-value="<%= strip_tags(projection_summary) %>"></div>
+             data-goal-projection-chart-aria-description-value="<%= strip_tags(@goal.projection_summary) %>"></div>
         <% if @goal.target_date.nil? %>
           <div class="mt-3 flex items-center gap-2 text-xs text-secondary">
             <span class="text-subdued"><%= icon("calendar-plus", size: "sm") %></span>