invoices: statistics page

Add /admin/invoices/stats with a year filter: invoiced, received and
outstanding totals in GEL at each invoice's captured rate (the one
currency every invoice has), overdue count, average days to pay, money
received per month by payment date as a bar chart with the same figures
in a table, per-currency totals, top clients and crypto option usage.
Cancelled invoices are left out of every amount; hidden ones follow the
usual superuser-only visibility.

Assisted-by: Claude:opus-5

views/admin/invoices/stats.erb

@@ -0,0 +1,98 @@
+<% s = @stats %>
+<% money = ->(v) { whole, frac = ('%.2f' % v).split('.'); "#{whole.reverse.scan(/\d{1,3}/).join(',').reverse.sub(/\A,/, '')}.#{frac}" } %>
+<% gel = ->(v) { "₾ #{money.(v)}" } %>
+<%== erb :'admin/_nav', layout: false %>
+<div class="header">
+    <h1>Invoice statistics</h1>
+    <p class="tagline"><a href="/admin/invoices">← invoices</a></p>
+</div>
+
+<nav class="year-filter" aria-label="Period">
+    <a href="/admin/invoices/stats" class="<%= 'active' unless s.year %>">all time</a>
+    <% s.years.each do |y| %><a href="/admin/invoices/stats?year=<%= y %>" class="<%= 'active' if s.year == y %>"><%= y %></a><% end %>
+</nav>
+
+<section class="section">
+    <div class="stats stats-6">
+        <div><b><%= gel.(s.gel(s.live)) %></b><span>invoiced</span></div>
+        <div><b><%= gel.(s.gel(s.paid)) %></b><span>received</span></div>
+        <div><b><%= gel.(s.gel(s.unpaid)) %></b><span>outstanding</span></div>
+        <div><b><%= s.overdue.size %></b><span>overdue</span></div>
+        <div><b><%= s.avg_days_to_pay ? "#{s.avg_days_to_pay} d" : '—' %></b><span>avg. time to pay</span></div>
+        <div><b><%= s.live.size %><% if s.cancelled.any? %> <small>+<%= s.cancelled.size %> cancelled</small><% end %></b><span>invoices</span></div>
+    </div>
+    <p class="optional">GEL at each invoice's captured rate. Cancelled invoices are excluded from all amounts.</p>
+</section>
+
+<% rows = s.monthly %>
+<% if rows.any? %>
+<section class="section">
+    <h2>Received per month <span class="optional">(₾, by payment date)</span></h2>
+    <% peak = rows.map(&:last).max %>
+    <% peak = BigDecimal('1') if peak.nil? || peak.zero? %>
+    <figure class="bar-chart" role="img" aria-label="Received per month in GEL; exact values in the table below">
+        <div class="bar-axis"><span><%= gel.(peak) %></span><span>0</span></div>
+        <div class="bars">
+            <% rows.each do |m, _n, _inv, rec| %>
+                <div class="bar-col" tabindex="0" data-tip="<%= m.strftime('%b %Y') %>: <%= gel.(rec) %>">
+                    <div class="bar" style="height: <%= (rec / peak * 100).round(1).to_f %>%"></div>
+                    <span class="bar-label"><%= rows.size > 12 ? m.strftime('%m/%y') : m.strftime('%b') %></span>
+                </div>
+            <% end %>
+        </div>
+    </figure>
+</section>
+
+<section class="section">
+    <h2>By month</h2>
+    <table class="invoice-list">
+        <thead><tr><th>Month</th><th class="num">Issued</th><th class="num">Invoiced ₾</th><th class="num">Received ₾</th></tr></thead>
+        <tbody>
+            <% rows.reverse_each do |m, n, inv, rec| %>
+                <% next if n.zero? && rec.zero? %>
+                <tr><td class="mono"><%= m.strftime('%Y-%m') %></td><td class="num mono"><%= n %></td><td class="num mono"><%= money.(inv) %></td><td class="num mono"><%= money.(rec) %></td></tr>
+            <% end %>
+        </tbody>
+    </table>
+</section>
+<% end %>
+
+<div class="dash">
+    <section class="section">
+        <h2>By currency</h2>
+        <table class="invoice-list">
+            <thead><tr><th>Currency</th><th class="num">Invoiced</th><th class="num">Received</th><th class="num">Outstanding</th></tr></thead>
+            <tbody>
+                <% s.by_currency.each do |cur, v| %>
+                    <tr><td class="mono"><%= cur %></td><td class="num mono"><%= money.(v[:invoiced]) %></td><td class="num mono"><%= money.(v[:paid]) %></td><td class="num mono"><%= money.(v[:outstanding]) %></td></tr>
+                <% end %>
+            </tbody>
+        </table>
+    </section>
+
+    <section class="section">
+        <h2>Top clients</h2>
+        <table class="invoice-list">
+            <thead><tr><th>Client</th><th class="num">Invoices</th><th class="num">Invoiced ₾</th></tr></thead>
+            <tbody>
+                <% s.top_clients.each do |client, n, g| %>
+                    <tr><td><%= client %></td><td class="num mono"><%= n %></td><td class="num mono"><%= money.(g) %></td></tr>
+                <% end %>
+            </tbody>
+        </table>
+    </section>
+
+    <% if s.crypto_usage.any? %>
+    <section class="section">
+        <h2>Crypto payment option</h2>
+        <table class="invoice-list">
+            <thead><tr><th>Asset</th><th class="num">Invoices</th></tr></thead>
+            <tbody>
+                <% s.crypto_usage.each do |coin, n| %>
+                    <tr><td><span class="mono"><%= coin %></span> <span class="optional"><%= CryptoAsset.name(coin) %></span></td><td class="num mono"><%= n %></td></tr>
+                <% end %>
+            </tbody>
+        </table>
+    </section>
+    <% end %>
+</div>