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/app_info.html

@@ -37,8 +37,8 @@
 <!-- Hero section -->
 <div class="app-info-hero">
     <img class="app-info-logo" src="{{logoURL .Meta.Slug}}"
-         alt="{{.Meta.DisplayName}}"
-         onerror="this.onerror=function(){this.style.display='none'};this.src='{{logoPNGURL .Meta.Slug}}'">
+         alt="{{.Meta.DisplayName}}" data-fallback="/static/app-placeholder.svg"
+         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 class="app-info-hero-text">
         {{if .AppInfo.Tagline}}
             <p class="app-info-tagline">{{.AppInfo.Tagline}}</p>