fix(lite): surface sync failures instead of a frozen "Syncing 99.9%"; don't report a lost server as "Synced"

Lite-parity audit, sync-progress semantics (state/presentation only — no send/sync backend call changed):
- A scan failure set scan->failed + lite_error, but no UI read lite_error, so the user saw "Syncing 99.9%"
  forever. The status bar now shows the error text on a lite scan/server failure.
- After the initial catch-up, a server loss on the cheap probe path (ProbeFailed) left the last "Synced"
  model on screen. The worker now re-publishes that model demoted (not complete, reason "server unreachable").
- refreshModelImpl's height-read "synced" override no longer fires while syncstatus says the backend is
  still scanning or is indeterminate, and a server-bound read failure in a full cycle demotes to not-synced.
- The syncstatus parser no longer treats a missing/unrecognised "syncing" field as fully synced: it reports
  an explicit "indeterminate" (not complete). Explicit "false" stays caught-up; the fake backend's
  syncing:true 1000/1000 behaviour is unchanged.

Tests: parser cases for missing/odd/idle "syncing" (testLiteSyncStatus*).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_phase4.cpp

@@ -12006,6 +12006,24 @@ void testLiteSyncStatusParserRealShapes()
         const auto p = parseLiteSyncStatusResponse(std::string("{\"syncing\":\"true\"}"));
         EXPECT_FALSE(p.ok);
     }
+    // Missing or unrecognised "syncing" must NOT default to fully synced.
+    {
+        const auto p = parseLiteSyncStatusResponse(std::string("{}"));
+        EXPECT_TRUE(p.ok);
+        EXPECT_FALSE(p.syncStatus.complete);
+        EXPECT_TRUE(p.syncStatus.indeterminate);
+        EXPECT_NEAR(p.syncStatus.progress, 0.0, 1e-9);
+        const auto q = parseLiteSyncStatusResponse(std::string("{\"syncing\":\"maybe\"}"));
+        EXPECT_TRUE(q.ok);
+        EXPECT_FALSE(q.syncStatus.complete);
+        EXPECT_TRUE(q.syncStatus.indeterminate);
+    }
+    // Explicit idle is still caught-up and determinate.
+    {
+        const auto p = parseLiteSyncStatusResponse(std::string("{\"syncing\":\"false\"}"));
+        EXPECT_TRUE(p.syncStatus.complete);
+        EXPECT_FALSE(p.syncStatus.indeterminate);
+    }
 }
 
 // Per-address balances: unspent notes/utxos are summed per address (spent ones excluded)