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Description

Fixes #563 — ONS names entered into the Add Moderator/Admin dialog were
immediately rejected with "This Account ID is invalid" instead of being
resolved to their corresponding 66-character hex public key.

Root cause: ModeratorsAddDialog.tsx validated input directly via
PubKey.from() with no prior ONS resolution step. The ONS resolver
(ONSResolve.getSessionIDForOnsName) already existed and was used by the
New Message flow (OverlayMessage.tsx), but was never called from the
moderator dialog.

What this PR does:

• Extracts a new shared helper resolvePubkeyOrOns() in
  ts/session/utils/resolvePubkeyOrOns.ts that encapsulates the full
  resolution ladder: trim/normalize -> pass through valid non-blinded 05
  pubkeys -> reject blinded/03-group keys -> ONS regex gate -> network
  resolve -> re-validate. Returns a discriminated union and never throws.
• Updates ModeratorsAddDialog.tsx to resolve each comma-separated entry
  through the shared helper before submission. Shows a spinner and disables
  the Add button during resolution. Surfaces failures inline via
  providedError rather than only a generic toast. Multi-add inputs name
  the specific failing entry.
• Refactors OverlayMessage.tsx (New Message flow) onto the same shared
  helper, collapsing ~45 lines of inline resolution logic into a single
  call. Behavior is fully preserved.
• Updates the Add Admins placeholder copy from accountId to
  accountIdOrOnsEnter so the dialog advertises ONS support.

Test approach:

• 11 new unit tests in ts/test/session/unit/utils/resolvePubkeyOrOns_test.ts
  covering: valid pubkey passthrough, whitespace trim, blinded key rejection,
  03-group key rejection, empty input, successful ONS resolution, dotted-name
  rejection, NotFoundError mapping, SnodeResponseError mapping, generic error
  mapping, and resolved-but-invalid-id handling.
• Full test suite: 919 passing, 0 failures.
• Manual testing on Windows 11 with SESSION_DEV=1: entering a valid ONS
  name in Add Admins now triggers async resolution; valid hex pubkeys still
  work unchanged; invalid inputs still show appropriate errors.
• OS: Windows 11