Make batch source search sequential per source and independent across sources

Refactor searchMissingBatch to schedule each source independently via a
per-source promise chain (scheduleSource) instead of processing all targets
in a single serial loop. This keeps source ordering intact while allowing
different sources to advance in parallel across models.

Also update searchOnline to accept scheduleSource and emit a pending UI
state for queued searches, split pending/running source colors in CSS, and
add tests covering independent batch source progression plus error/cancel
survival.

web/css/resolver-main.css

@@ -1103,14 +1103,20 @@
     --mr-source-border-alpha: 0.22;
 }
 
-.mr-missing-source-running,
-.mr-missing-source-pending {
+.mr-missing-source-running {
     --mr-source-rgb: 96, 190, 255;
     --mr-source-fill-alpha: 0.16;
     --mr-source-border-hi-alpha: 0.78;
     --mr-source-border-alpha: 0.6;
 }
 
+.mr-missing-source-pending {
+    --mr-source-rgb: 175, 180, 188;
+    --mr-source-fill-alpha: 0.10;
+    --mr-source-border-hi-alpha: 0.52;
+    --mr-source-border-alpha: 0.38;
+}
+
 .mr-missing-source-error {
     --mr-source-rgb: 255, 138, 128;
     --mr-source-fill-alpha: 0.16;
@@ -3675,6 +3681,10 @@
     display: none;
 }
 
+.mr-search-progress-pending .mr-search-progress-fill {
+    background: var(--mr-text-muted);
+}
+
 .mr-footer:not(.mr-footer-is-missing) .mr-footer-star-link {
     margin-right: auto;
 }