Streams: list / switch / merge down / copy up

- Tools -> Streams: list server streams, switch the workspace to a stream,
  merge down from parent / copy up to parent (into a pending changelist)
- Backend p4 streams, client -s -S, merge/copy -S

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

src/App.css

@@ -780,6 +780,7 @@ body.resizing-v{cursor:row-resize!important;user-select:none}
 .srch-list{flex:1;overflow-y:auto;padding:6px 10px 12px}
 .srch-row{display:flex;align-items:center;gap:8px;padding:8px 10px;border-radius:9px;transition:background .12s}
 .srch-row:hover{background:var(--hover)}
+.srch-row.on{background:rgba(124,110,246,.12);box-shadow:inset 0 0 0 1px rgba(124,110,246,.25)}
 .srch-body{flex:1;display:flex;flex-direction:column;gap:1px;min-width:0}
 .srch-body .n{font-size:13px;font-weight:600;white-space:nowrap;overflow:hidden;text-overflow:ellipsis}
 .srch-body .p{font-size:11px;color:var(--faint);white-space:nowrap;overflow:hidden;text-overflow:ellipsis}