Invites and pairing polish #386

Follow-ups from v2.7.0 batch B (#384, KodeStar/audiosilo-bridge#70, KodeStar/audiosilo-frontend#297).

Polish left on the invite and pairing work and on the LibriVox console after batch B (some continue #380).

• Certificate warning misses self-signed names. The invite dialog warns "Their browser will warn about the certificate" only for https at an IP address (admin-ui/src/features/people/people-model.ts:191, isIPHost). A self-signed name such as nas.local gets no warning. Key the warning on the certificate the server uses (self-signed TLS mode), not on the host's shape.
• Phone invite lead for a typed code. The invite page on a phone says "Install AudioSilo, then open this invite link again and tap Open in app." (internal/web/assets/i18n-dict.js:31, index.html:36), which reads oddly to someone who reached it by typing a code rather than following a link.
• Second Share press. Pressing Share again while the system share sheet is still open falls back to copying underneath the open sheet (admin-ui/src/lib/clipboard.ts:71). Ignore the press while a share is in flight.
• Failed and cancelled jobs lose their last step in Settings > Bridge > Jobs. For those jobs the last step is the clue to where it stopped, so keep showing it (a done job should not, as batch B made it).
• metaID vs model.ValidSlug. The LibriVox fetch checks ids with its own metaID pattern (internal/bridge/librivox.go:47) while contribute and upcoming use model.ValidSlug (internal/bridge/contribute.go:168). Use one rule.
• The Adopted tri-state is unpacked by hand (internal/catalog/librivox.go:63, :131). A small helper or named state would read better.
• The player's has_cover. The server keeps books.has_cover (internal/catalog/books.go:137) but the player is not told it. LibriVox add: console and notification polish #380 saw a book whose audio has no cover still get a cover_version, so the player asks for /libraries/{id}/cover, gets a 404 and logs a console error. Give the player a has_cover (additive) or stop sending cover_version without a cover.
• Two console refactors skipped in batch B: a shared console failureCode, and moving useTrackedJob (admin-ui/src/features/librivox/use-librivox.ts:43) into the bridge feature.