feat(studio): header and inspector tabs on the shared primitives (#3624)

* feat(studio): header and inspector tabs on the shared primitives

The header's view-mode toggle and the inspector's tab strip become real
Tabs, so both gain the roving tabindex, arrow keys and Home/End they never
had; undo, redo and Inspector become IconButton and Button; Export becomes
the primary Button at the medium size. Capture stays an <a download>,
wearing Button's recipe, because a button cannot save a file.

Colour literals in StudioHeader.tsx drop to zero: the brand gradient moves
into theme.css as two tokens and the Export's hand-written text colour
becomes text-bg-0. Ratchet total 475 to 470.

* docs(studio): shorten the header and inspector tab comments to fit the comment gate

* chore(studio): bank the lower colour-literal baseline for StudioHeader

* fix(studio): arrow keys only move focus when the inspector strip toggles panes

* chore(studio): allow deleting PanelTabButton, replaced by the shared Tabs primitive

scripts/check-no-main-deletions.mjs

@@ -44,6 +44,10 @@ export const ALLOWED_DELETIONS = new Map([
     "packages/studio/src/components/StudioGlobalDragOverlay.tsx",
     "the full-screen drop-to-add box is replaced by a landing preview on the timeline; no remaining references",
   ],
+  [
+    "packages/studio/src/components/PanelTabButton.tsx",
+    "replaced by the shared Tabs primitive in RightPanelTabs; no remaining references",
+  ],
   [
     "packages/studio/postcss.config.js",
     "Tailwind v4 migration: postcss replaced by @tailwindcss/vite",