fix: resolve QG-001 typography token drift and QG-006 style violations

Replace raw line-height values with --leading-none/--leading-loose
design tokens in 3 CSS files. Fix 7 PHP style issues: import ordering,
unused imports, brace placement, multi-line argument formatting.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/css/components/app-icon-button.css

@@ -13,7 +13,7 @@
     background: transparent;
     color: var(--app-muted-color);
     font-size: var(--text-sm);
-    line-height: 1;
+    line-height: var(--leading-none);
     cursor: pointer;
     text-decoration: none;
     transition:
@@ -29,6 +29,6 @@
 
   .app-icon-button i {
     font-size: 15px; /* icon-font metric — intentional px */
-    line-height: 1;
+    line-height: var(--leading-none);
   }
 }

web/css/components/app-tabs.css

@@ -47,7 +47,7 @@
     background: transparent;
     color: var(--tabs-muted);
     font-size: var(--text-xs);
-    line-height: 1;
+    line-height: var(--leading-none);
     width: 24px;
     height: 24px;
     padding: 0;

web/css/layout/app-shell.css

@@ -12,7 +12,7 @@
 
   small {
     --app-font-size: 0.875em;
-    line-height: 1.9;
+    line-height: var(--leading-loose);
   }
 
   div#debugger-bar {