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>

README.md

@@ -181,7 +181,8 @@ Saved connections live in a plain JSON file in the OS app-data directory
 `Space` select · `Enter` open folder · `Backspace` parent ·
 `Ctrl/Cmd+A` select all · `F5` send to other panel · `F2` rename ·
 `F7` new folder · `F8`/`Delete` delete · `Cmd/Ctrl+T` new tab ·
-`Cmd/Ctrl+W` close tab · `Ctrl+Tab` next tab.
+`Cmd/Ctrl+W` close tab · `Ctrl+Tab` next tab ·
+`Cmd/Ctrl+=`/`-`/`0` interface bigger / smaller / 100%.
 
 (On Macs, function keys may need `fn` — every action also has a toolbar button.)
 
@@ -220,7 +221,7 @@ application menu ([src/bun/menu.ts](src/bun/menu.ts)) is macOS-only for now;
 the in-app UI covers every menu action.
 
 Right-click menus are **HTML, on every platform**
-([ui/src/app/context-menu.ts](ui/src/app/context-menu.ts)) rather than native —
+([ui/src/app/core/context-menu.service.ts](ui/src/app/core/context-menu.service.ts)) rather than native —
 electrobun's native context menu is a documented no-op on Linux, and one menu
 implementation beats two that drift. Only reveal / open-with-default / copy-path
 cross to the backend, through the `shellItemAction` RPC.