Stage 1: make this branch Windows 7 only

The dual-skin arrangement existed to let one build toggle between the modern
shadcn look and 7.css. Keeping both correct is what forced the cascade tricks
that fight 7.css, so the branch drops the toggle: this build IS the Windows 7
build.

- `skin` is gone from AppSettings/DEFAULT_SETTINGS, the settings store, the
  SettingsService (signal, applySkin, persist payload), the options popover
  control, and the i18n keys.
- buildPreload adds `.win7` unconditionally; `chromeOptions()` no longer takes a
  skin and always returns frameless `titleBarStyle: "hidden"`.
- `IS_WIN7_CHROME` and `IS_MAC_CHROME` are deleted along with every
  `isMacChrome` branch in the tab bar, editor window and connection manager.
  Those reserved `pl-24` for macOS traffic lights, which frameless chrome
  removes on every platform, so the branches were dead by construction.
- The title bar is no longer conditional — it is the window's only chrome.
- index.html keeps just the win7 anti-flash arm (four theme states).

Not done here, deliberately: stripping Tailwind's element styling. The
neutralizers in styles.css (notably `.win7 button { min-width: 0 }`) are still
load-bearing until each component's markup is converted to 7.css's real
contract, so they come out per component rather than in one sweep.

Verified: backend `tsc --noEmit` clean, `bun run build:ui` clean; bundle
490.50 kB, down from 492.52 kB.

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

ui/src/app/tab-bar/tab-bar.ts

@@ -5,12 +5,11 @@ import { LOCALE_LABELS, SUPPORTED } from '@shared/i18n';
 import type { AccentTone } from '@shared/types';
 import { I18nService } from '../i18n.service';
 import { PanelsService } from '../panels.service';
-import { IS_MAC_CHROME, IS_WIN7_CHROME, MOD_LABEL } from '../platform';
+import { MOD_LABEL } from '../platform';
 import { RpcService } from '../rpc.service';
 import { SettingsService } from '../settings.service';
 import { UpdaterService } from '../updater.service';
 
-const SKINS = ['modern', 'win7'] as const;
 const THEMES = ['system', 'light', 'dark'] as const;
 const DARK_TONES = ['dark', 'darker'] as const;
 // Swatch colors are fixed mid-tones so every dot reads on both light and dark
@@ -43,10 +42,7 @@ const CONCURRENCY_CHOICES = [2, 3, 4, 6, 8] as const;
          traffic lights, and draggable. Elsewhere it's a normal compact row. -->
     <div
       class="w7-toolbar relative flex items-center gap-0.5 border-b border-border bg-chrome pr-1.5 electrobun-webkit-app-region-drag"
-      [class.h-10]="isMacChrome"
-      [class.pl-24]="isMacChrome"
-      [class.pl-1.5]="!isMacChrome"
-      [class.pt-1]="!isMacChrome"
+      class="h-9 pt-1 pl-1.5"
     >
       <div
         #tabScroll
@@ -181,30 +177,6 @@ const CONCURRENCY_CHOICES = [2, 3, 4, 6, 8] as const;
             </select>
           </div>
 
-          <div class="mt-2.5 flex items-center justify-between gap-2">
-            <span class="text-xs text-muted-foreground">{{ i18n.t('options.skin.label') }}</span>
-            <div class="flex rounded-md border border-border p-0.5">
-              @for (option of skins; track option) {
-                <button
-                  (click)="settings.setSkin(option)"
-                  class="rounded px-2 py-0.5 text-[11px]"
-                  [class.bg-accent]="settings.skin() === option"
-                  [class.text-foreground]="settings.skin() === option"
-                  [class.text-muted-foreground]="settings.skin() !== option"
-                >
-                  {{ i18n.t('options.skin.' + option) }}
-                </button>
-              }
-            </div>
-          </div>
-          <!-- The CSS half of the skin flips live; the window frame does not
-               (titleBarStyle is fixed at construction — see chromeOptions). -->
-          @if (settings.skin() !== bootSkin) {
-            <p class="mt-1 text-right text-[10px] text-muted-foreground">
-              {{ i18n.t('options.skin.restartHint') }}
-            </p>
-          }
-
           <div class="mt-2.5 flex items-center justify-between gap-2">
             <span class="text-xs text-muted-foreground">{{ i18n.t('options.theme.label') }}</span>
             <div class="flex rounded-md border border-border p-0.5">
@@ -457,14 +429,11 @@ export class TabBar {
   readonly settingsOpen = signal(false);
   readonly localeOptions = SUPPORTED;
   readonly localeLabels = LOCALE_LABELS;
-  readonly isMacChrome = IS_MAC_CHROME;
   readonly modLabel = MOD_LABEL;
   readonly appName = BRANDING.appName;
   readonly buildLabel = BRANDING.buildLabel;
-  readonly skins = SKINS;
   /** What the window was BUILT with, not the current setting — the frameless
    *  chrome only changes on reopen, so the hint compares against this. */
-  readonly bootSkin: (typeof SKINS)[number] = IS_WIN7_CHROME ? 'win7' : 'modern';
   readonly themes = THEMES;
   readonly darkTones = DARK_TONES;
   readonly accents = ACCENTS;