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>

src/bun/index.ts

@@ -15,13 +15,7 @@ import { migrateLegacyDataDir } from "./store/paths";
 import { SettingsStore } from "./store/settings";
 import { AppTray } from "./tray";
 import { TransferEngine } from "./transfers/engine";
-import {
-	buildPreload,
-	chromeOptions,
-	initSessionSkin,
-	showWhenReady,
-	trackMaximizeState,
-} from "./windows";
+import { buildPreload, chromeOptions, showWhenReady } from "./windows";
 
 const PROD_URL = "views://app/index.html";
 
@@ -60,19 +54,7 @@ const events = {
 	openConnectionManager: () => {},
 	showAbout: () => {},
 };
-// Late-bound for the same reason events is: `win` below takes this rpc in its
-// constructor, so it cannot exist yet. Only the win7 title bar reads it.
-let mainWin: BrowserWindow | undefined;
-const rpc = createAppRPC({
-	registry,
-	engine,
-	store,
-	settings,
-	edits,
-	watcher,
-	mainWindow: () => mainWin,
-	events,
-});
+const rpc = createAppRPC({ registry, engine, store, settings, edits, watcher, events });
 // late-bound like events: a watched local dir changed -> tell the main webview
 watcher.onChange = (dir) => rpc.send.pathRefresh({ sessionId: "local", dir });
 const url = await getMainViewUrl();
@@ -82,11 +64,6 @@ const url = await getMainViewUrl();
 // fresh via the saveSettings flow below.
 let currentSettings = await settings.load();
 setBackendLocale(currentSettings.locale); // native menus/errors follow the UI language
-// Freeze the skin for this process BEFORE any window is constructed. It picks
-// a component tree and a native frame, neither of which can change on a live
-// window, so the persisted value may only take effect at the next start — see
-// initSessionSkin.
-initSessionSkin(currentSettings.skin);
 
 const win = new BrowserWindow({
 	title: BRANDING.appName,
@@ -97,8 +74,7 @@ const win = new BrowserWindow({
 		x: 120,
 		y: 120,
 	},
-	// Frame decision lives in chromeOptions — see windows.ts. Constructor-only,
-	// and read from the pinned session skin, not from currentSettings.
+	// frame decision lives in chromeOptions (windows.ts); constructor-only
 	...chromeOptions(),
 	// lets the UI pick the fused layout + theme before first paint
 	preload: buildPreload("main", currentSettings),
@@ -107,8 +83,6 @@ const win = new BrowserWindow({
 	hidden: true,
 });
 showWhenReady(win);
-mainWin = win;
-trackMaximizeState(win, (maximized) => rpc.send.windowState({ maximized }));
 
 const editors = new EditorWindows({
 	registry,