Balances: one reservation rule (earned first) + purchased-grade credited pool

- balances(): live campaign budgets are set aside up front, earned pool first then
  purchased, so dashboard numbers never move per impression; new 'in campaigns' line
- earned pool gains a purchase_grade portion for refunds/comps: shows under Purchased
  as 'credited to you' and funds login ads (daily login charge draws on it first)
- viewing credits always spend before credited purchased money
- dashboard/donut/spend banner/earn pane/positions show purchased, credited, earned, in campaigns
- chatbot canned answer + AI prompt: the balance rule

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/my.html

@@ -399,6 +399,7 @@
           <div class="lbl">Available to spend</div>
           <div class="big"><span id="spendBig">0</span> <span class="unit">credits</span></div>
           <div class="sub" id="spendSub"></div>
+          <div class="sub small" id="spendNote"></div>
           <div class="rates" id="spendRates"></div>
         </div>
         <div class="card" id="whereCard">