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/platform-notes.md

@@ -8,9 +8,7 @@ macOS is the primary target; Windows 11+ x64 and Linux x64 are **beta** targets
 
 | Concern | File | mac | win | linux |
 |---|---|---|---|---|
-| Window chrome (modern skin) | `src/bun/windows.ts`, `index.ts` | hiddenInset + traffic-light offset, UI drag header | native titlebar, compact header | same as win |
-| Window chrome (win7 skin) | `src/bun/windows.ts chromeOptions` | `hidden` (clears Titled → no traffic lights) | frameless | `gtk_window_set_decorated(false)` |
-| Window **resize** | `ui/.../7like/window-resize/` | native frame resizes; win7 frameless uses the synthesized handles | as mac | as mac |
+| Window chrome | `src/bun/windows.ts chromeOptions` | hiddenInset + traffic-light offset, UI drag header | native titlebar, compact header | same as win |
 | External editor launch | `src/bun/edits.ts launchEditor` | `open [-a app]` | `cmd /c start "" [app] file` | app directly / `xdg-open` |
 | Editor app picker | `rpc-handlers.ts pickEditorApp` | /Applications, `.app` | %ProgramFiles%, `.exe` | home, any |
 | Owner/group names | `fs/local.ts resolveId` | `id -nu` + `dscacheutil` | undefined → "—" | `id -nu` + `getent` |
@@ -25,7 +23,7 @@ macOS is the primary target; Windows 11+ x64 and Linux x64 are **beta** targets
 | ssh-agent | `fs/sftp.ts` | $SSH_AUTH_SOCK | `\\.\pipe\openssh-ssh-agent` fallback | $SSH_AUTH_SOCK |
 | Paths in UI | `ui format.ts joinPath/isRootPath` | "/" | separator detected from the dir string; drive-root regex | "/" |
 
-The **context menu is HTML on every platform** (`ui/src/app/skins/*/context-menu.ts` +
+The **context menu is HTML on every platform** (`ui/src/app/skins/shadlike/context-menu.ts` +
 `context-menu.service.ts`), not a native electrobun menu — see the Linux section
 below for why. Only the three shell actions it can't do itself (reveal, open with
 the default app, copy path) cross to bun, via the `shellItemAction` RPC.
@@ -33,29 +31,6 @@ the default app, copy path) cross to bun, via the `shellItemAction` RPC.
 Remote (SFTP) paths are ALWAYS POSIX regardless of host or server OS
 (`path.posix` throughout `fs/sftp.ts`).
 
-### Frameless windows lose the WM's resize borders (win7 skin, every platform)
-
-`titleBarStyle: "hidden"` is what lets 7.css draw the Aero caption — on Linux the
-GTK wrapper honours it with `gtk_window_set_decorated(false)`. Undecorating also
-removes the window manager's resize borders and corner grips, and electrobun
-1.18.1 offers **no** native resize gesture to put back: the shipped
-`libNativeWrapper` exports `startWindowMove`/`stopWindowMove` with no resize
-counterpart, does not link `gtk_window_begin_resize_drag`, and `BrowserWindow`
-has neither `setResizable` nor min-size constructor options. The window is still
-*programmatically* resizable (`setFrame`/`getFrame` work, and
-`gtk_window_set_resizable(false)` is never called), so the app **synthesizes the
-input affordance**: a bottom-right grip in the webview drags the native frame
-over RPC (`ui/src/app/skins/7like/window-resize/`). Only that one corner — an
-invisible handle anywhere else overlays a control and steals its hover, which is
-what a full edge set did to the caption buttons.
-
-That is a **stopgap** — each drag tick is an RPC hop plus an FFI call, where a
-real WM grab is compositor-side. `scripts/check-electrobun-resize.ts` watches the
-installed electrobun for the missing primitives and prints the removal checklist
-when they appear; it runs from `postinstall`, so an electrobun bump surfaces it
-without anyone remembering to look. The modern skin keeps the native frame and
-needs none of this.
-
 ## Memory footprint (expected, not a bug)
 
 Investigated 2026-07 — packaged build. Roughly **~200 MB macOS / ~300 MB