Stage 1: extract the skin-agnostic UI layer into app/core/

First step of splitting the UI into two skin trees. All 12 services plus
platform.ts, format.ts, file-icons.ts, icons.ts and app.config.ts move to
ui/src/app/core/ — the layer both skins will share.

This is a pure move: git records all 17 as renames, and no file content
changed beyond import paths. The premise holds because `git diff
main...feat/7css-skin` touches none of these files — the service layer was
already skin-agnostic, so there is nothing to untangle, only to relocate.

Components now reach it through a new `@core/*` tsconfig alias rather than a
relative path. That is deliberate and load-bearing for the next stage: skin
components are about to move under app/skins/<skin>/ at varying depths, and
an alias keeps every one of these 67 imports correct wherever its file ends
up. `@shared/*` (the bun-side src/shared) already set the precedent.

Intra-core imports needed no edits at all — every ./sibling reference among
the moved files points at another moved file.

Verified: tsc clean, `bun run build:ui` clean (AOT resolves every import and
template symbol), and the running window's opaque interior is pixel-identical
to a pre-refactor capture.

Note on that comparison: the Aero frame and title bar do NOT compare equal
between launches, and that is not a regression — .window.glass::before runs
at opacity .6, so the chrome composites over whatever desktop is behind the
window and shifts by a uniform ~8% brightness. The check therefore diffs the
opaque interior (inset 30px sides, 80px top), where anything this refactor
could break would show.

Co-Authored-By: Claude <noreply@anthropic.com>

ui/src/main.ts

@@ -1,9 +1,9 @@
 import { bootstrapApplication } from '@angular/platform-browser';
-import { appConfig } from './app/app.config';
+import { appConfig } from '@core/app.config';
 import { App } from './app/app';
 import { ConnectionManager } from './app/connection-manager/connection-manager';
 import { EditorWindow } from './app/editor-window/editor-window';
-import { APP_MODE } from './app/platform';
+import { APP_MODE } from '@core/platform';
 
 // One bundle, three window kinds: the preload global picks the root component.
 const root = APP_MODE === 'editor' ? EditorWindow : APP_MODE === 'manager' ? ConnectionManager : App;