Warn loudly when a <wpd-*> tag is used without being imported

[AllTerrainDeveloper]

Why

A <wpd-*> tag that appears in the DOM without a matching
customElements.define() renders as an inert generic element — no
shadow root, no styles, no behavior, and no error. That silent
failure is the single most common reason a developer's UI "looks
broken for no reason," and the fix (one missing side-effect import)
is invisible from devtools.

This PR turns that silent failure into a loud, actionable
console.error so the next developer who hits it loses minutes, not
hours.

What it does

A new always-on warner watches the document and every open shadow
root. For any unregistered <wpd-*> tag that doesn't get upgraded
within ~2 seconds, it logs one of three messages:

1. Known tag, never imported — prints the exact
   import '<rel>/ui/components/<name>/<name>'; line to paste.
2. Typo (e.g. <wpd-buton>) — "Did you mean <wpd-button>?"
   via Levenshtein distance against the canonical tag list.
3. Unknown tag — points at the registry so the dev can
   discover real names.

Each error attaches the offending element so devtools'
inspect-element jumps straight to it. One warning per tag per page —
not spammy.

How to test

Open OS Settings → Components. You'll see a new
"Missing-import warner — live demo" section. The console shows:

• A styled yellow banner explaining the next three errors are
  intentional.
• Three console.error entries, one per branch above.

If you want the demo gone from your fork, delete the section in
src/settings/sections/help.ts — the warner itself stays.

Scope

• src/ui/components/missing-import-warner.ts — new module.
• src/desktop.ts — starts the warner before init().
• src/settings/sections/help.ts — adds the demo section + banner.

No public API changes. No behavior changes for end users.

[AllTerrainDeveloper]

[github-actions]

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