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>

src/shared/i18n/en.ts

@@ -37,12 +37,6 @@ export const en = {
 		title: "Options",
 		language: "Language",
 		system: "System",
-		skin: {
-			label: "Skin",
-			modern: "Modern",
-			win7: "Windows 7",
-			restartHint: "Reopen the window to change its frame",
-		},
 		theme: { label: "Theme", system: "System", light: "Light", dark: "Dark" },
 		tone: { label: "Tone", dark: "Dark", darker: "Darker" },
 		accent: { label: "Accent" },