Launcher polish: monogram reveal-on-failure + app placeholder on every icon surface (v0.163.1)

(A) The launcher monogram bled through every transparent white logo — it rendered
unconditionally under the img. Now hidden by default (.launch-mono display:none),
revealed only when the img chain fails (onerror adds .launch-tile--noimg).

(B) The /static/app-placeholder.svg default reached only app_list_row. The four
other sanctioned app-logo onerror chains now match the canonical SVG->PNG->placeholder
grammar: backups_apps (aligned row), stacks (infra->infra-logo else placeholder),
app_info hero (screenshots still vanish), deploy (keeps .LogoURL/.LogoPNGURL).

Template/CSS only; no handler/funcmap change. 5 tests + 2 red-proofs.

controller/internal/web/templates/stacks.html

@@ -22,7 +22,7 @@
     <div class="stack-detail-card stack-state-{{stateColor .State}}" data-filter-state="{{filterCategory .State .Deployed}}"{{if .Meta.Slug}} data-href="/apps/{{.Meta.Slug}}"{{end}}>
         <div class="stack-detail-header">
             <div class="stack-title-row">
-                <img class="stack-logo-lg" src="{{logoURL .Meta.Slug}}" alt=""{{if $im}} data-fallback="/static/infra-logo.svg"{{end}}
+                <img class="stack-logo-lg" src="{{logoURL .Meta.Slug}}" alt="" data-fallback="{{if $im}}/static/infra-logo.svg{{else}}/static/app-placeholder.svg{{end}}"
                      onerror="if(!this.dataset.step){this.dataset.step='1';this.src='{{logoPNGURL .Meta.Slug}}';}else if(this.dataset.fallback&&this.dataset.step==='1'){this.dataset.step='2';this.src=this.dataset.fallback;}else{this.onerror=null;this.style.visibility='hidden';}">
                 <div>
                     <h3>{{if $im}}{{$im.DisplayName}}{{else}}{{.Meta.DisplayName}}{{end}}</h3>