Size every icon on TDesign's scale: four rem tokens, whole pixels

TDesign draws its glyphs on a 24px grid with a 2px stroke and sizes a
component's icon 2px above its text, in whole pixels. The templates had
drifted to 12 one-off sizes, most of them fractional (0.9rem = 14.4px), so
strokes blurred and neighbours never matched: the tab bar's 14.4px cog
beside a 12.8px +, the shortcut rows' 12px + beside an 11.2px reset and
9.6px chip x.

Every <ng-icon> now takes one of --icon-size-sm (14px: beside 11-12px
text, small icon buttons), -md (16px: file rows, window titles, path-bar
buttons), -lg (20px) or -xl (24px: lone spinners and status marks).
--action-icon goes; architecture.md and gotchas.md carry the rule.

Co-Authored-By: Claude <noreply@anthropic.com>

ui/src/app/skins/shadlike/app.html

@@ -2,7 +2,7 @@
   @if (!rpc.available) {
     <div class="flex flex-1 items-center justify-center">
       <div class="max-w-sm border border-border bg-card p-6 text-center shadow-sm">
-        <ng-icon name="tdesignCloseCircle" size="1.5rem" class="text-destructive" />
+        <ng-icon name="tdesignCloseCircle" size="var(--icon-size-xl)" class="text-destructive" />
         <p class="mt-2 text-sm font-medium">{{ i18n.t('app.notConnectedTitle') }}</p>
         <p class="mt-1 text-xs text-muted-foreground">
           {{ i18n.t('app.notConnectedBody') }}
@@ -28,7 +28,7 @@
       (mousedown)="focusManager()"
     >
       <div class="border border-border bg-card px-5 py-4 text-center shadow-xl">
-        <ng-icon name="tdesignServer" size="1.25rem" class="text-muted-foreground" />
+        <ng-icon name="tdesignServer" size="var(--icon-size-lg)" class="text-muted-foreground" />
         <p class="mt-1.5 text-xs text-muted-foreground">{{ i18n.t('app.managerOpen') }}</p>
         <button
           (click)="focusManager()"