Remove the Windows 7 (7.css) skin

The win7 skin is dropped. Everything else the branch built stays: Linux
support and the HTML context menus, the workspace-tab menu, the continuous
interface scale (rem everywhere, stepper, Ctrl/Cmd +/-/0) and the core/
service layer. The modern tree already carried all of them, and nothing
outside skins/7like/ imported the win7 tree except main.ts and styles.css.

Removed: the skins/7like/ tree (5.1k lines, fonts), the 7.css dependency,
the skin setting and its options row + strings in all 7 locales, the
frameless-window plumbing (window-control RPCs, windowState push,
trackMaximizeState, the synthesized resize grip and its postinstall
watchdog), the session-skin pinning in buildPreload/chromeOptions, the
skin/skinfix cascade layers, and the win7-only probes and screenshot
harness (the harness cannot render the modern tree: plain Firefox has no
RPC bridge). chromeOptions() stays as the one shared frame decision.

Fixed on the way out:
- index.html's per-theme pre-paint background is restored. The branch had
  replaced it with a win7-only transparent rule, so modern dark windows had
  silently lost one of the three flash guards.
- editor-windows.ts wrote its window-key separator as a literal NUL byte,
  which made git and grep treat the file as binary; it is `\0` now (same
  string at runtime).

No settings migration: a stale "skin" key is carried harmlessly by the
store's spread-merge and dropped on the next save.

UI bundle: 616.7 kB -> 405.4 kB, back under the 500 kB budget.

Docs: architecture, gotchas and platform-notes lose the win7 material but
keep what stays true (construction-time preloads/frames, drag regions
ignoring preventDefault, electrobun's missing resize/maximize primitives);
flash-prevention, dev-URL and Bun-panic notes move back under Electrobun;
stale context-menu/tab-bar links fixed; README documents the scale keys.

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

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

@@ -6,12 +6,11 @@ import type { AccentTone } from '@shared/types';
 import { ContextMenuService } from '@core/context-menu.service';
 import { I18nService } from '@core/i18n.service';
 import { PanelsService, type Workspace } from '@core/panels.service';
-import { APP_SKIN, IS_MAC_CHROME, MOD_LABEL } from '@core/platform';
+import { IS_MAC_CHROME, MOD_LABEL } from '@core/platform';
 import { RpcService } from '@core/rpc.service';
 import { SettingsService } from '@core/settings.service';
 import { UpdaterService } from '@core/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
@@ -182,36 +181,6 @@ const CONCURRENCY_CHOICES = [2, 3, 4, 6, 8] as const;
             </select>
           </div>
 
-          <!-- Skin: a LAUNCH-TIME choice. It swaps the whole component tree
-               (ui/src/app/skins/) and the native frame, and neither can change
-               on a live window — the tree is bootstrapped from the preload-baked
-               __APP_SKIN and titleBarStyle/transparent are BrowserWindow
-               constructor options. Clicking only saves; the running UI is left
-               alone. Highlight the SAVED value so the click registers, and show
-               the pending hint whenever it differs from the booted skin. -->
-          <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 (s of skins; track s) {
-                <button
-                  (click)="settings.setSkin(s)"
-                  [title]="i18n.t('options.skin.restartHint')"
-                  class="rounded px-2 py-0.5 text-[0.6875rem]"
-                  [class.bg-accent]="settings.skin() === s"
-                  [class.text-foreground]="settings.skin() === s"
-                  [class.text-muted-foreground]="settings.skin() !== s"
-                >
-                  {{ i18n.t('options.skin.' + s) }}
-                </button>
-              }
-            </div>
-          </div>
-          @if (skinPending()) {
-            <p class="mt-1 text-right text-[0.625rem] 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">
@@ -484,13 +453,6 @@ export class TabBar {
   readonly modLabel = MOD_LABEL;
   readonly appName = BRANDING.appName;
   readonly buildLabel = BRANDING.buildLabel;
-  readonly skins = SKINS;
-  /**
-   * The saved skin differs from the one this window actually booted, so the
-   * choice is waiting on a restart. APP_SKIN is the preload-baked value that
-   * picked the running component tree; settings.skin() is merely persisted.
-   */
-  readonly skinPending = computed(() => this.settings.skin() !== APP_SKIN);
   readonly themes = THEMES;
   readonly darkTones = DARK_TONES;
   readonly accents = ACCENTS;