refactor: modular route organization (Phase 1-2 complete)

- Split monolithic build route handler into focused modules
- Extract validation, multi-copy, include/exclude, themes, and partner routes
- Add response utilities and telemetry decorators
- Create route pattern documentation
- Fix multi-copy detection bug (tag key mismatch)
- Improve code maintainability and testability

Roadmap 9 M1 Phase 1-2

code/web/templates/build/_new_deck_modal.html

@@ -1137,7 +1137,7 @@
           .then(function(r){ return r.text(); })
           .then(function(html){ slot.innerHTML = html; })
           .catch(function(){ slot.innerHTML = ''; });
-      }catch(_){ }
+      }catch(e){ }
     }
     // Listen for OOB updates to the tags slot to trigger fetch
     document.body.addEventListener('htmx:afterSwap', function(ev){

code/web/templates/build/_new_deck_multicopy.html

@@ -4,7 +4,7 @@
   <div class="muted" style="font-size:12px; margin-bottom:.35rem;">We detected a viable multi-copy archetype for your commander/themes. Choose one or skip.</div>
   <div style="display:grid; gap:.5rem;">
     {% for it in items %}
-    <label class="mc-option" style="display:grid; grid-template-columns: auto 1fr; gap:.5rem; align-items:flex-start; padding:.5rem; border:1px solid var(--border); border-radius:8px; background:#0b0d12;">
+    <label class="mc-option" style="display:grid; grid-template-columns: auto 1fr; gap:.5rem; align-items:flex-start; padding:.5rem; border:1px solid var(--border); border-radius:8px; background:var(--panel);">
       <input type="radio" name="multi_choice_id" value="{{ it.id }}" {% if loop.first %}checked{% endif %} />
       <div>
         <div><strong>{{ it.name }}</strong> {% if it.printed_cap %}<span class="muted">(Cap: {{ it.printed_cap }})</span>{% endif %}</div>