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/tsconfig.json

@@ -4,8 +4,14 @@
   "compileOnSave": false,
   "compilerOptions": {
     "strict": true,
+    /* @shared -> the bun/UI shared code (src/shared). @core -> the skin-agnostic
+       UI layer (services, platform, formatters). @core is an ALIAS rather than a
+       relative path on purpose: skin components live at varying depths under
+       app/skins/<skin>/, and an alias keeps their imports identical wherever a
+       component is moved to. */
     "paths": {
-      "@shared/*": ["../src/shared/*"]
+      "@shared/*": ["../src/shared/*"],
+      "@core/*": ["./src/app/core/*"]
     },
     "noImplicitOverride": true,
     "noPropertyAccessFromIndexSignature": true,