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>

docs/build-and-release.md

@@ -119,7 +119,7 @@ Wired on top of the download host above. See the "Auto-updater" section in
   `applyUpdate` (handlers in [rpc-handlers.ts](../src/bun/rpc-handlers.ts) call the electrobun `Updater`);
   push message `updateStatus`.
 - **UI**: `UpdaterService` ([ui/src/app/core/updater.service.ts](../ui/src/app/core/updater.service.ts)) holds signals
-  (`state`/`version`/`progress`); the tab-bar "update pill" ([tab-bar.ts](../ui/src/app/skins/7like/tab-bar/tab-bar.ts))
+  (`state`/`version`/`progress`); the tab-bar "update pill" ([tab-bar.ts](../ui/src/app/skins/shadlike/tab-bar/tab-bar.ts))
   is one accent (`bg-primary`) control that announces → fills as a progress bar while downloading →
   "Restart to apply". `state` is `available → downloading → ready` (ready derived from the
   `downloadUpdate` response). App-menu "Check for Updates…" ([menu.ts](../src/bun/menu.ts)) routes a