Document the core/skins split

architecture.md gains a "UI layout: one core, two skin trees" section — the
folder map, the barrel contract that lets main.ts stay ignorant of both trees,
why duplicate selectors across trees are safe, and where a new rule belongs
(core / a skin / shared tokens). Its "Window kinds" section now describes
two-axis root selection, and the "Skins" section is no longer stale prose about
a deleted design: it covers the stylesheet split and the two-phase nature of a
skin change (CSS live, tree and frame at next window open).

gotchas.md gains a section for the split, all of it earned during the work:
`.win7` cannot be renamed because 7.scoped.css is precompiled against that
literal; reach core through @core/* rather than relative paths, which is what
let 67 imports survive two relocations; @font-face url() rebases when a
stylesheet moves and fails silently; the @layer statement must stay first;
duplicate selectors across trees are safe; and ngtsc type-checks components
nothing imports, so a green build does say something about the inactive skin.

Also corrected the component paths that moved under skins/, and the
updater.service.ts link that moved under core/. Checked every ../ link in
docs/ resolves.

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

docs/gotchas.md

@@ -31,10 +31,10 @@ touching the adjacent code.
 - **The win7 skin layer outranks Tailwind utilities.** 7.css is imported
   `layer(skin)`, ordered above `utilities`, so any utility you add to a
   `button`/`input`/`select`/`textarea`/`table` is silently dead under that
-  skin. If it matters, neutralize in the `@layer skin` block at the bottom of
-  styles.css — and note `revert-layer` only reaches `utilities` from **inside
-  `skin`**; used from `skinfix` it falls back onto 7.css and is a no-op. This
-  bit once already.
+  skin. If it matters, neutralize in the `@layer skin` block in
+  `app/skins/7like/7like.css` — and note `revert-layer` only reaches
+  `utilities` from **inside `skin`**; used from `skinfix` it falls back onto
+  7.css and is a no-op. This bit once already.
 - `@import … layer(name)` works through `@tailwindcss/postcss` (it wraps the
   imported AST), but the **deep path is mandatory**: 7.css ships no `exports`
   map and Tailwind's resolver reads only `style`/`mainFields`, so a bare
@@ -54,6 +54,33 @@ touching the adjacent code.
   it** — not even in an HTML comment. A backtick in a comment terminates the
   literal and the errors point at the prose, not at the quoting.
 
+## The two skin trees (`ui/src/app/skins/`)
+
+- **`.win7` is not a name we may change.** `7.scoped.css` is precompiled with
+  every selector prefixed by that literal class, so the skin id in
+  `AppSettings` stays `"win7"` even though its folder is `7like/`. The folder
+  names are cosmetic; the class and the persisted setting value are not.
+- **Reach `core/` through the `@core/*` alias, never a relative path.** Skin
+  components sit at varying depths and get moved; the alias is what let 67
+  imports survive two relocations untouched. A `../../core/...` path is a
+  future merge conflict.
+- **`@font-face` `url()` resolves relative to the file that declares it.** When
+  the win7 rules moved three directories deeper, `./fonts/...` would silently
+  have 404'd — build still green, app just renders a fallback font. The fonts
+  therefore live inside the skin that uses them.
+- **The `@layer` statement must stay the first statement in `ui/src/styles.css`.**
+  It is one of only two rule types allowed above `@import`, and it is what sorts
+  `skin` above `utilities`. Every `@import` (Tailwind and both skin sheets) must
+  also precede all other rules.
+- **Duplicate selectors across skins are safe.** `app-file-panel` exists in both
+  trees; standalone components resolve through their importer's `imports:`
+  array, never a global registry, and the trees are never mounted together.
+- **The build DOES type-check components nothing imports.** ngtsc compiles the
+  whole program, so a skin that is currently unreachable still fails
+  `bun run build:ui` on a type or template error. Verified deliberately by
+  injecting an error into an unreferenced file — worth knowing before assuming
+  a green build proves nothing about the inactive skin.
+
 ## The win7 skin (7.css)
 
 - **Unlayered declarations beat every `@layer`, regardless of specificity.**
@@ -113,7 +140,7 @@ touching the adjacent code.
   `dist/api/bun/proc/linux.md` says so; the app also prints "Application menus are
   not supported on Linux" at boot). Because the UI suppresses the webview's own
   right-click menu, that left EVERY panel context menu dead on Linux — no error,
-  nothing happens. The menu is plain HTML now (`ui/src/app/context-menu.ts`) on
+  nothing happens. The menu is plain HTML now (`ui/src/app/skins/*/context-menu.ts`) on
   every platform; don't reintroduce the native one for "nativeness" on mac/win
   without keeping a Linux path.