Keep showing a guest link's URL so it can be copied again

The URL was shown once, in a box under the create button, and then gone: only
a hash of the token was stored, so the app genuinely could not produce it a
second time. Lose the message you sent the sitter and the only way back was to
mint a new link — which strands whoever is already holding the old one.

Settings now lists every live link with its URL and a Copy button, so re-sending
one is just copying it again.

That means keeping the token rather than only its hash, and it is worth being
plain about the trade. It is not the trade you would make for a password, which
the user has probably reused, or a session token, which grants everything
indefinitely. A guest link grants a strict subset of what the same database
already holds in plaintext, expires on a date the owner picked, and can be
revoked in one tap — so an attacker who can read puppy.db gains very little by
also being able to open it as a guest. The lookup column stays a hash and
remains the key redeem matches against; the secret sits in a new column beside
it, which also keeps the migration additive.

Links created before this have an empty secret. They keep working and stay
revocable — the migration touches nothing but the new column — and the list
says why their URL is missing rather than rendering a broken one.

The two tests that asserted the old contract now assert the new one: a listing
hands back a secret that really opens the link, and the lookup column is still
a hash. Added one for the legacy row, since "still works, just cannot be shown"
is the part a future change is most likely to break quietly.

server/main.go

@@ -377,6 +377,7 @@ func openDB(path string) (*sql.DB, error) {
 			id        TEXT PRIMARY KEY,
 			user_id   TEXT NOT NULL,
 			token     TEXT NOT NULL UNIQUE,
+			secret    TEXT NOT NULL DEFAULT '',
 			label     TEXT NOT NULL DEFAULT '',
 			created   INTEGER NOT NULL,
 			expires   INTEGER NOT NULL,
@@ -476,6 +477,18 @@ func migrateSchema(db *sql.DB) error {
 			return err
 		}
 	}
+	// Guest links are re-showable in Settings, which means keeping the token
+	// itself and not only its hash (see Auth.createShare). Links made before
+	// this have an empty secret and simply cannot be shown again.
+	hasSecret, err := columnExists(db, "share_links", "secret")
+	if err != nil {
+		return err
+	}
+	if !hasSecret {
+		if _, err := db.Exec(`ALTER TABLE share_links ADD COLUMN secret TEXT NOT NULL DEFAULT ''`); err != nil {
+			return err
+		}
+	}
 	hasShareID, err := columnExists(db, "sessions", "share_id")
 	if err != nil {
 		return err