fix(claw): read a blank since as unset instead of failing the request

The phone's notification poll sent `since` unconditionally, and before
the first ack there is no cursor at all, so it went out as `?since=`.
Axum's `Query` hands a present-but-empty parameter through as an empty
string, and `Option<i64>` only forgives a *missing* key, so the whole
poll answered `400 Failed to deserialize query string: since: cannot
parse integer from empty string` and the phone showed an error instead
of an empty queue.

Both ends: the client omits `since` unless it has a positive id to send,
and a `blank_as_none` deserializer makes a blank value mean "not set"
for `NotificationQuery` and `EventStreamQuery` alike — an empty `since`
reads as "from the start" however it got there.

android/app/src/main/java/rs/rhema/phone/net/ClawApi.kt

@@ -138,12 +138,21 @@ class ClawApi(
         return execute(builder.delete().build()) { }
     }
 
+    // GET /v1/notifications?device=&since=N. `since` is the last id the
+    // device has acked; before the first ack there is no cursor at all,
+    // and an empty `since=` came back
+    // "HTTP 400 — Failed to deserialize query string: since: cannot
+    // parse integer from empty string", which failed the whole poll.
+    // So the parameter is sent only when there is a positive id to send:
+    // absent and blank mean the same thing to the claw, and the shorter
+    // request is the honest one.
     suspend fun notifications(deviceId: String, since: String): ClawResult<List<RhemaNotification>> {
         val base = baseOrNull() ?: return ClawResult.Network("no base URL configured")
+        val cursor = since.trim().toLongOrNull()?.takeIf { it > 0 }
         val url = base.toHttpUrlOrNull()?.newBuilder()
             ?.addPathSegments("v1/notifications")
             ?.addQueryParameter("device", deviceId)
-            ?.addQueryParameter("since", since)
+            ?.apply { cursor?.let { addQueryParameter("since", it.toString()) } }
             ?.build()
             ?: return ClawResult.Network("bad base URL")
         val request = Request.Builder().url(url).header("Authorization", bearer()).build()