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/core/context-menu.service.ts

@@ -16,8 +16,8 @@ export interface MenuItem {
   tooltip?: string;
   /** Shortcut hint, rendered right-aligned. */
   accelerator?: string;
-  /** Greyed and unclickable. Win7 menus disable rather than hide, so the set
-   *  of commands stays in the same place from one right-click to the next. */
+  /** Greyed and unclickable. Menus disable rather than hide, so the set of
+   *  commands stays in the same place from one right-click to the next. */
   disabled?: boolean;
 }
 
@@ -133,7 +133,7 @@ export class ContextMenuService {
    * assume `activeWorkspace`.
    *
    * Everything illegal is DISABLED, not omitted: a menu whose items move
-   * between right-clicks is a Win7 sin, and "Close Tab" greyed out explains
+   * between right-clicks is worse than a dead row, and "Close Tab" greyed out explains
    * why the last tab cannot be closed better than its absence does.
    *
    * Duplicate/Rename/Save-session are deliberately absent until tabs have an