fix: two regressions this rebuild introduced for non-APRS users

Both found by an auditor comparing behaviour against the released build rather than
against the intent of the change.

Prefix-first portable callsigns were rejected. CallsignEntry took the first segment -
`call.substringBefore('/')` - and required a letter and a digit in it. A portable call can
be written prefix-first, DL/W1AW or ZL/JA1ABC or OH/W1AW/MM, where the leading token is a
country prefix with no digit at all. Measured: five such forms were refused where the old
length-only check had accepted them. Any segment may now carry the callsign.

JSON cookie exports stopped working for QRZ. QrzGridParser.cookieHeader converts the JSON
array a browser extension produces into a Cookie header, and it had zero production
callers - the raw pasted text went straight into the header. The old client normalised it.
So an operator whose export had been working would see their cookie sent as a literal JSON
blob, QRZ would serve its signed-out page, and the app would tell them the cookie had
expired when it was perfectly good. A raw `k=v; k=v` paste was unaffected, which is why
this survived review.

Both are cases where a rewrite lost behaviour the old code had. Neither had a test.

core/data/src/main/java/com/rtbishop/look4sat/core/data/qrz/QrzGridSource.kt

@@ -66,7 +66,19 @@ class QrzGridSource(
     }
 
     /** Fetch [url], retrying transport failures with backoff. Null when every attempt failed. */
-    private suspend fun fetchWithRetry(url: String, cookieHeader: String): String? {
+    /**
+     * Normalise whatever the operator pasted into a Cookie header value.
+     *
+     * They paste either a raw `k=v; k=v` header or the JSON array a cookie-export extension
+     * produces. The old client normalised this and the rewrite dropped it, so a JSON export that
+     * used to work went out as a literal JSON blob, QRZ served its signed-out page, and the app
+     * told the operator their cookie had expired when it was perfectly good.
+     */
+    private fun normalise(raw: String): String = QrzGridParser.cookieHeader(raw)
+
+    private suspend fun fetchWithRetry(url: String, rawCookie: String): String? {
+        val cookieHeader = normalise(rawCookie)
+        if (cookieHeader.isBlank()) return null
         repeat(MAX_ATTEMPTS) { attempt ->
             try {
                 val request = Request.Builder().url(url)