Stage 4: restore the modern tree into app/skins/shadlike/

Both UIs now live side by side. The 18 modern component files come straight
from `git show main:<path>` — recovered, not rewritten, which is possible
because main's tip IS the merge base of this branch. There is no title-bar
among them: that component is win7-only, since the modern skin defers to the
native window frame.

Their imports were rewritten to @core the same way stage 1 did, leaving only
skin-internal relative imports behind. Added skins/shadlike/index.ts exporting
SHAD_ROOTS with the same shape as SEVEN_ROOTS, so main.ts can select a tree
without knowing either skin's internals.

Restored the skin axis in core/platform.ts, deleted in fe028a9 and needed
again by the modern tree: __APP_SKIN on the Window global, APP_SKIN,
IS_WIN7_CHROME, and IS_MAC_CHROME = IS_MAC && !IS_WIN7_CHROME. That last one
is what reserves pl-24 for the macOS traffic lights in the fused hiddenInset
header — and correctly stops reserving it under win7, where titleBarStyle
'hidden' removes the traffic lights entirely.

The tree is not reachable yet; the build is the gate. I checked that the gate
is real rather than assuming it: injecting a type error into an unreferenced
shadlike file does fail `bun run build:ui`, so ngtsc type-checks the whole
program including components nothing imports. Without that check, "the build
passes" would have proven nothing about 18 files no one references.

win7 remains pixel-identical to the pre-refactor baseline.

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

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

@@ -0,0 +1,58 @@
+<div class="flex h-screen flex-col bg-background text-foreground">
+  @if (!rpc.available) {
+    <div class="flex flex-1 items-center justify-center">
+      <div class="max-w-sm rounded-lg border border-border bg-card p-6 text-center shadow-sm">
+        <ng-icon name="mynaXCircle" size="1.5rem" 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') }}
+        </p>
+      </div>
+    </div>
+  } @else {
+    <app-tab-bar />
+
+    <main class="flex min-h-0 flex-1 divide-x divide-border">
+      <app-file-panel side="left" />
+      <app-file-panel side="right" />
+    </main>
+
+    <app-transfer-bar />
+    <app-status-bar />
+  }
+
+  <!-- modal block: the connection-manager window owns the app until closed -->
+  @if (managerOpen()) {
+    <div
+      class="fixed inset-0 z-60 flex items-center justify-center bg-black/40"
+      (mousedown)="focusManager()"
+    >
+      <div class="rounded-lg border border-border bg-card px-5 py-4 text-center shadow-xl">
+        <ng-icon name="mynaServers" size="1.25rem" class="text-muted-foreground" />
+        <p class="mt-1.5 text-xs text-muted-foreground">{{ i18n.t('app.managerOpen') }}</p>
+        <button
+          class="mt-3 rounded-md bg-primary px-3 py-1.5 text-xs font-medium text-primary-foreground hover:bg-primary/90"
+        >
+          {{ i18n.t('app.showConnections') }}
+        </button>
+      </div>
+    </div>
+  }
+
+  <!-- dialogs -->
+  @if (dialogs.prompt(); as req) {
+    <app-prompt-dialog [request]="req" />
+  }
+  @if (dialogs.confirm(); as req) {
+    <app-confirm-dialog [request]="req" />
+  }
+  @if (dialogs.chmod(); as req) {
+    <app-chmod-dialog [request]="req" />
+  }
+  @if (dialogs.overwrite(); as req) {
+    <app-overwrite-dialog [request]="req" />
+  }
+
+  <app-context-menu />
+  <app-toasts />
+</div>