feat: VITE_DEV_TOOLS env-gate for preview tooling on deployed builds

Adds a useDevTools() composable that returns true when either Vite's
own DEV flag is set (local pnpm dev) or VITE_DEV_TOOLS='true' is
passed at build time. AppLayout's PaletteSwitcher now reads through
this composable, so the picker can render on earthwalker.aiolabs.dev
without flipping the build to development mode.

Single gate intentionally — future preview-only surfaces (debug
overlays, unfinished pages, work-in-progress UI) hook into the same
composable. Anything Nicholette should see but the public shouldn't
becomes a v-if="devTools" check. Set the var on the deploy host,
rebuild, surfaces appear; unset, surfaces vanish.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

env.d.ts

@@ -5,6 +5,10 @@ interface ImportMetaEnv {
   readonly VITE_OWNER_NPUB?: string
   /** Comma-separated wss:// relay list; falls back to a default set if unset. */
   readonly VITE_NOSTR_RELAYS?: string
+  /** When set to 'true', in-progress preview tooling (the palette
+   * picker today, future debug overlays / unfinished pages) renders
+   * in any build. Gated via useDevTools(). */
+  readonly VITE_DEV_TOOLS?: string
 }
 
 interface ImportMeta {