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>

ui/src/styles.css

@@ -1,21 +1,4 @@
-/* Layer order — declared BEFORE the imports so it wins (a @layer statement is
-   one of the only two rule types allowed above @import). `skin` sorts after
-   `utilities`, so every 7.css rule outranks every Tailwind utility on the
-   elements it targets; that inversion is what lets the win7 skin restyle
-   ~1900 inline utility classes without touching the templates. `skinfix` is
-   above both, and uses `revert-layer` to hand individual properties back to
-   `utilities` where 7.css's Win7-dialog metrics are wrong for this app.
-   Tailwind re-declares the first four names, a no-op once ordered here.
-   See the "Windows 7 skin" section at the bottom of this file. */
-@layer theme, base, components, utilities, skin, skinfix;
 @import "tailwindcss";
-/* The two skin stylesheets. Both ship in one bundle and are scoped by the
-   `.win7` class on <html>, exactly as when they lived in this file — only one
-   skin's rules can match at a time. They sit here because CSS requires every
-   @import to precede all other rules; layer assignment is fixed by the @layer
-   statement above, not by this order. */
-@import "./app/skins/shadlike/shadlike.css";
-@import "./app/skins/7like/7like.css";
 
 /* shadcn-style theme tokens. Light stays the zinc family; the dark neutrals are a
    warm gray family (hue ~107) anchored on #1a1a19 — see the dark blocks below.
@@ -192,9 +175,8 @@
 /* Panel-chrome metrics: file list, path bar, action buttons, source picker.
    ALL REM, deliberately — the interface scale (settings.uiScale) works by
    setting the root font-size, so every rem value in the app scales from that
-   one lever. A px value here would freeze while everything around it grew,
-   which is exactly what happened to the win7 boxes before stage 1c. There is
-   only one tier now: the old data-text compact/large tiers are gone, replaced
+   one lever. A px value here would freeze while everything around it grew.
+   There is only one tier now: the old data-text compact/large tiers are gone, replaced
    by the continuous scale. Values are the old default tier verbatim
    (13px -> 0.8125rem etc. at the 16px root), so scale 1.0 is pixel-identical
    to the pre-scale app. */