missions: collapsible header + scrollable tabs + wrap phase controls

- Mission header title/description now collapsible via chevron next
  to the title. Persisted in localStorage so it stays hidden across
  mission switches once the operator has read it — clears more room
  for phases/tasks/team panels below.
- Tabs row: overflow-x auto + per-tab flex:none + whiteSpace:nowrap
  so 8+ tabs scroll horizontally instead of wrapping and cutting off.
- Phase card action row (retry/security/benchmark buttons): flexWrap
  wrap so long button rows stack cleanly instead of overflowing.
- Phase card status row wraps too, and the card itself gets
  overflow:hidden + minWidth:0 so long content stays inside the
  border and the parent tab-panel scroll handles vertical growth.

frontend/src/components/dashboard/MissionCanvas.tsx

@@ -9,7 +9,17 @@
 // This replaces ResearchCanvas + LoopsCanvas after Slice 9's cutover.
 
 import React, { useCallback, useEffect, useMemo, useState } from "react";
-import { FileText, Pencil, Play, Plus, RefreshCw, Sparkles, Trash2 } from "lucide-react";
+import {
+  ChevronDown,
+  ChevronUp,
+  FileText,
+  Pencil,
+  Play,
+  Plus,
+  RefreshCw,
+  Sparkles,
+  Trash2,
+} from "lucide-react";
 
 import {
   deleteMission,
@@ -120,6 +130,21 @@ export function MissionCanvas({
   const [deleteBusy, setDeleteBusy] = useState(false);
   const [runs, setRuns] = useState<MissionRunSummary[]>([]);
   const [lastLoadedAt, setLastLoadedAt] = useState<Date | null>(null);
+  // Persist the collapsed state across mission switches so the operator
+  // can keep the header hidden once they've read it.
+  const [headerCollapsed, setHeaderCollapsed] = useState<boolean>(() => {
+    if (typeof window === "undefined") return false;
+    return window.localStorage.getItem("cm.mission.headerCollapsed") === "1";
+  });
+  const toggleHeader = useCallback(() => {
+    setHeaderCollapsed((v) => {
+      const next = !v;
+      if (typeof window !== "undefined") {
+        window.localStorage.setItem("cm.mission.headerCollapsed", next ? "1" : "0");
+      }
+      return next;
+    });
+  }, []);
 
   const load = useCallback(async () => {
     if (!selectedId) {
@@ -495,8 +520,26 @@ export function MissionCanvas({
             })()}
           </div>
         </div>
-        <h1 style={{ margin: 0, fontSize: 20, color: "#f3f3f5" }}>{mission.title}</h1>
+        <div style={{ display: "flex", alignItems: "center", gap: 8 }}>
+          <h1 style={{ margin: 0, fontSize: 20, color: "#f3f3f5", flex: 1, minWidth: 0 }}>
+            {mission.title}
+          </h1>
           {mission.description && (
+            <button
+              type="button"
+              onClick={toggleHeader}
+              title={headerCollapsed ? "Expand description" : "Collapse description"}
+              aria-label={headerCollapsed ? "Expand description" : "Collapse description"}
+              style={{
+                ...iconBtn,
+                flex: "none",
+              }}
+            >
+              {headerCollapsed ? <ChevronDown size={13} /> : <ChevronUp size={13} />}
+            </button>
+          )}
+        </div>
+        {mission.description && !headerCollapsed && (
           <div
             style={{
               marginTop: 4,
@@ -539,7 +582,16 @@ export function MissionCanvas({
             }}
           />
         )}
-        <div style={{ display: "flex", gap: 4, marginTop: 4 }}>
+        <div
+          style={{
+            display: "flex",
+            gap: 4,
+            marginTop: 4,
+            overflowX: "auto",
+            paddingBottom: 2,
+            scrollbarWidth: "thin",
+          }}
+        >
           {(
             [
               "overview",
@@ -582,6 +634,8 @@ export function MissionCanvas({
                   display: "inline-flex",
                   alignItems: "center",
                   gap: 6,
+                  flex: "none",
+                  whiteSpace: "nowrap",
                 }}
               >
                 {t}
@@ -641,15 +695,26 @@ export function MissionCanvas({
                     display: "flex",
                     flexDirection: "column",
                     gap: 4,
+                    minWidth: 0,
+                    overflow: "hidden",
+                  }}
+                >
+                  <div
+                    style={{
+                      display: "flex",
+                      alignItems: "center",
+                      gap: 8,
+                      flexWrap: "wrap",
+                      minWidth: 0,
                     }}
                   >
-                  <div style={{ display: "flex", alignItems: "center", gap: 8 }}>
                     <span
                       style={{
                         width: 7,
                         height: 7,
                         borderRadius: "50%",
                         background: PHASE_STATUS_COLOR[p.status],
+                        flex: "none",
                       }}
                     />
                     <span
@@ -686,7 +751,14 @@ export function MissionCanvas({
                     <PhaseSummaryCard missionId={mission.id} phaseId={p.id} />
                   )}
                   {mission.status === "running" || mission.status === "completed" ? (
-                    <div style={{ display: "flex", gap: 6, marginTop: 6 }}>
+                    <div
+                      style={{
+                        display: "flex",
+                        gap: 6,
+                        marginTop: 6,
+                        flexWrap: "wrap",
+                      }}
+                    >
                       {p.status === "failed" && mission.status === "running" && (
                         <button
                           type="button"