feat(approvals,audit): record the reviewer as the unchecked claim it is

The hub copies the reviewer string a client sends straight into
decided_by (DecideApproval, ClearEventLog); nothing on the wire ties
it to the authenticated caller. Studio filled it from the OS account,
so an export read like non-repudiation while being an arbitrary
client claim — the legal finding of the 2026-07-26 usertest panel.

The real fix is hub-side (derive decided_by from CALLER_IDENTITY);
that contract is written down in docs/reviewer-identity.md and needs
a hub release. Until then Studio does the one thing it can do
honestly and marks its own claim as a claim, inside the record:

- data/reviewer_identity.dart is the single place that produces and
  reads the value; wire() is idempotent, so page and HubService may
  both normalise. Every write path funnels through HubService, so no
  surface can send a bare handle.
- The inbox states before the decision who will be recorded, what
  that attribution is worth on this hub (from AuthStatus), and the
  literal string that lands in decided_by. An unreadable auth policy
  stays unreadable — never optimistic.
- Reading back: a marked value shows its plain name plus an
  unchecked flag; an unmarked one (legacy row, CLI decision, or a
  future hub-derived identity) is not classified either way.
- The audit wipe seeds the same kind of marked attribution into its
  chain.reset marker.

When the hub starts deriving the value it overwrites the field and
the prefix disappears by itself — no Studio release needed.

Guards: reviewer_identity_test (the value) and
approvals_reviewer_identity_test (every surface that writes or
renders it, against the hermetic fake hub). Visual proof for both
themes via the dialog-shot harness.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

CHANGELOG.md

@@ -52,6 +52,24 @@ Approvals usertest-panel hardening:
   (not "PROMPT"), and the no-data hint drops developer jargon.
 - **Copy the run id in one click**, and the history payload renders
   with the same JSON pretty-printing as the card.
+- **A reviewer name no longer poses as a proven identity.** The hub
+  stores whatever `reviewer` string a client sends as `decided_by`,
+  so Studio's `$USER@studio` read like non-repudiation while being
+  an unchecked claim (legal finding of the 2026-07-26 panel). Every
+  value Studio writes — approvals and the `chain.reset` marker of
+  the audit wipe — now carries an `unverified:` marker inside the
+  record, so an export, a CLI reader or a database dump sees the
+  trust level without knowing how the deciding Studio was set up.
+  The inbox states before the decision who will be recorded and
+  what that is worth on this hub (derived from the hub's auth
+  policy; "unreadable" stays unreadable, never optimistic). Reading
+  back, a marked value shows its plain name with an "unchecked"
+  flag, an unmarked one (legacy row, CLI decision) is not
+  classified either way. The real fix is hub-side — derive
+  `decided_by` from the authenticated caller; the contract is in
+  `docs/reviewer-identity.md`, and once the hub does it the marker
+  disappears without a Studio release. Guards:
+  `reviewer_identity_test` + `approvals_reviewer_identity_test`.
 - **Audit overflow-menu entries no longer run off the menu.** A
   popup menu is width-capped, so the full-sentence German labels
   were clipped — on the reset entry exactly the "(nur local/dev)"