feat(marketplace-ui): show whether an app's authorship was actually proven

The backend verifies DID signatures as of f0c289a4, but the card only rendered
`trust_tier` / `trust_score`, so the verdict reached the frontend and died
there. Adds a badge next to the existing trust pill.

Deliberately a *separate* badge rather than folding into the trust tier: the
score blends relay count, provenance and policy compliance, while this answers
one narrow question — did the author prove control of the key their `author.did`
names. Merging them would hide the distinction that the signature layer exists
to draw.

- `valid`   → green "signed" with a lock glyph
- `missing` → neutral grey "unsigned" (an unsigned publisher is unproven, not
              hostile, so it reads as absence rather than alarm)
- `invalid` → red "bad signature". Discovery drops these before they reach the
              cache, so it should be unreachable; rendered anyway so the UI
              fails visibly rather than silently if that ever changes.

Two fail-safe details:
- The mapping defaults a missing field to `{status:'missing'}` rather than
  leaving it undefined. A node on an older backend returns no field at all, and
  "we couldn't check" must never render as "signed".
- The `invalid` arm is typed in the RPC client for the same reason: an
  unhandled status falls through to "unsigned", not to the green badge.

The tooltip carries the meaning the two-word badge can't. "Signed" is easy to
misread as "safe", so it says what was actually proven — who published it — and
explicitly that this is not a statement about the app being safe.

Verified: vue-tsc clean, build green, and the new strings are present in the
freshly built Marketplace chunk (the build can silently no-op).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

neode-ui/src/api/rpc-client.ts

@@ -1084,6 +1084,16 @@ class RPCClient {
       relay_count: number
       first_seen: string
       nostr_pubkey: string
+      /**
+       * Whether the author proved control of the key their `author.did` names.
+       * `invalid` manifests are dropped during discovery and should never
+       * appear here; typed anyway so the UI fails safe rather than falling
+       * through to "signed" if that ever changes.
+       */
+      signature?:
+        | { status: 'valid' }
+        | { status: 'missing' }
+        | { status: 'invalid'; reason: string }
     }>
     relay_count: number
   }> {