Stage 3: split styles.css into a shared base plus per-skin sheets

The 919-line stylesheet had a clean seam: `main`'s version is 278 lines and
this branch only ever APPENDED to it. Verified that directly — a diff of
main's styles.css against our first 298 lines differs only by the @layer
statement and the 7.css import.

- ui/src/styles.css (296) keeps the shared half: the @layer statement, the
  Tailwind import, the OKLCH tokens, @theme inline, the data-text interface
  sizes, the animations and the accent axis. The @layer statement must stay
  first here — it is one of only two rule types allowed above @import, and it
  is what sorts `skin` above `utilities`.
- app/skins/7like/7like.css (636) takes the whole win7 section plus the
  scoped-7.css import that feeds layer(skin).
- app/skins/shadlike/shadlike.css is near-empty by nature, not by omission:
  that skin IS the shared token system plus inline Tailwind, so there is no
  framework to bridge and no cascade to invert. The file documents why, and
  what does/does not belong in it.

Both are @imported from styles.css, so one bundle still ships both, scoped by
`.win7` exactly as before.

One real trap caught here: the @font-face url() is resolved relative to the
stylesheet, so moving the win7 section three directories deeper would have
silently 404'd Noto Sans — the build stays green and the app just falls back
to a different font. Fixed by moving the fonts INTO the skin (they exist only
for the Win7 look, where Noto Sans substitutes for Segoe UI), which keeps
`./fonts/...` correct wherever the folder goes. Confirmed in the built output:
both woff2 emitted as hashed assets with the url rewritten to ./media/.

Verified: build clean, built CSS keeps `@layer theme,base,components,
utilities,skin,skinfix` and all 7.css selectors, the live window's opaque
interior is pixel-identical to the pre-refactor baseline, and both harness
renders (light and dark) diff to exactly zero. A layer-assignment mistake here
would not be subtle — 7.css would lose to ~1900 Tailwind utilities and the app
would look nothing like this.

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

ui/src/app/skins/shadlike/shadlike.css

@@ -0,0 +1,27 @@
+/* ==========================================================================
+   shadlike skin — the modern, shadcn-style look
+   ==========================================================================
+
+   Intentionally near-empty, and that is not an oversight.
+
+   This skin IS the shared foundation: the OKLCH token system, `@theme inline`,
+   the data-text interface sizes and the accent axis all live in ui/src/styles.css
+   because both skins consume them, and everything else the modern look needs is
+   a Tailwind utility written inline in its templates. There is no third-party
+   framework to bridge and no cascade inversion to undo, so there is simply
+   nothing left for a skin-level stylesheet to say.
+
+   Compare app/skins/7like/7like.css, which is 630 lines: all of it exists to
+   import 7.css above `utilities`, re-point the app's semantic tokens at Win7
+   values, and hand individual properties back where 7.css's dialog metrics are
+   wrong for this app.
+
+   This file exists so the folder is self-contained and symmetrical with the
+   other skin — modern-only rules that should not leak into the win7 cascade
+   have a home here. Rules that BOTH skins need belong in ui/src/styles.css
+   instead; anything added here is dead weight under .win7.
+
+   Scoping note: 7like scopes itself with the `.win7` class that 7.scoped.css
+   requires. This skin is the default (no marker class), so any rule added here
+   must NOT be written so broadly that it also matches under .win7 — prefer
+   scoping to the component, not to bare elements. */