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>

bun.lock

@@ -21,7 +21,6 @@
       "name": "ui",
       "version": "0.0.0",
       "dependencies": {
-        "7.css": "^0.21.1",
         "@angular/common": "^22.0.0",
         "@angular/compiler": "^22.0.0",
         "@angular/core": "^22.0.0",
@@ -46,8 +45,6 @@
     "@parcel/watcher",
   ],
   "packages": {
-    "7.css": ["7.css@0.21.1", "", {}, "sha512-Q+/aFUpGemoR/Zltj4a7yvHonyxfb+9cgiwBL+j/Lm+H28EOoLzWoJki7MdGPV+3mLCvKSCVqr3M2x2uNYyeWw=="],
-
     "@algolia/abtesting": ["@algolia/abtesting@1.18.0", "", { "dependencies": { "@algolia/client-common": "5.52.0", "@algolia/requester-browser-xhr": "5.52.0", "@algolia/requester-fetch": "5.52.0", "@algolia/requester-node-http": "5.52.0" } }, "sha512-8siuLG+FIns1AjZ/g2SDVwHz9S+ObacDQISEJvS8XsNei1zl3FXqfqQrBpmrG7ACWCyesXHbicMJtvRbg00FEw=="],
 
     "@algolia/client-abtesting": ["@algolia/client-abtesting@5.52.0", "", { "dependencies": { "@algolia/client-common": "5.52.0", "@algolia/requester-browser-xhr": "5.52.0", "@algolia/requester-fetch": "5.52.0", "@algolia/requester-node-http": "5.52.0" } }, "sha512-wtwPgyPmO7b7sQPVgoK29c1VpfS08DnnJCmxX/oU1pV2DlMRJCzQcLN7JSloYpodyKHwM8+9wOzlAM0co3TDmA=="],