Fill in empty states for run detail Arguments, Output, and History sections

The run-detail page always renders Arguments, Output (when closed), and
History even when those sections have no data to show. Previously they
rendered as an empty vue-json-pretty "null" node or an empty history
viewport with just tab buttons, leaving operators to guess whether the
run genuinely has nothing or whether the page failed to load something.

This replaces the silent blanks with explicit, operator-facing messages:

- Arguments: "This run was started without arguments." when the decoded
  payload is null/empty/empty collection.
- Output: "This run completed without returning a value." when a closed
  run has no output payload.
- History: "No history events have been recorded for this run yet."
  when the run has no timeline events, displayed in place of the empty
  virtual-scroll timeline or the empty event-list table.

A new hasPayload() helper decodes the raw argument/output value through
the existing unserialize() path, then treats null, undefined, empty
string, empty array, and empty object as "no payload". Sections that are
already gated on their own length (Activities, Tasks, Waits, Timers,
Commands, Signals, Updates, Linked Intakes, Exceptions) keep their
existing behaviour — those cards stay hidden when empty.

Author: durable-workflow-ops

public/app.js

@@ -1,5 +1,5 @@
 {
-    "/app.js": "/app.js?id=147cf2116110d73e68bbbf8ca6b4562d",
+    "/app.js": "/app.js?id=86372e1a1c7bb44d2caa2bc89cf0c13f",
     "/app-dark.css": "/app-dark.css?id=64d24ee96944ffdb4b26a9be1658c1e3",
     "/app.css": "/app.css?id=d525454610dfd3c5a581fc49676f8a37",
     "/img/favicon.png": "/img/favicon.png?id=7c006241b093796d6abfa3049df93a59",

public/mix-manifest.json

@@ -2,7 +2,7 @@
     <div>
         <div class="card">
             <div class="card-header d-flex align-items-center justify-content-between">
-                <h5 v-if="!ready">Run Detail</h5>
+                <h5 v-if="!ready">Flow Preview</h5>
                 <h5 v-if="ready">{{ flow.class }}</h5>
 
                 <div class="d-flex align-items-center">
@@ -81,7 +81,7 @@
                 class="d-flex flex-column align-items-center justify-content-center text-center card-bg-secondary p-5 bottom-radius"
                 role="alert"
                 aria-live="assertive">
-                <strong>Run detail unavailable</strong>
+                <strong>Flow preview unavailable</strong>
                 <span class="text-muted mt-2">{{ loadingError }}</span>
                 <button class="btn btn-outline-primary btn-sm mt-3" @click="retryFlowLoad">
                     Retry
@@ -499,7 +499,10 @@
             </div>
 
             <div class="card-body code-bg text-white collapse show" id="collapseArguments">
-                <vue-json-pretty :data="unserialize(flow.arguments)"></vue-json-pretty>
+                <vue-json-pretty v-if="hasPayload(flow.arguments)" :data="unserialize(flow.arguments)"></vue-json-pretty>
+                <div v-else class="text-muted small">
+                    This run was started without arguments.
+                </div>
             </div>
         </div>
 
@@ -514,7 +517,10 @@
             </div>
 
             <div class="card-body code-bg text-white collapse show" id="collapseOutput">
-                <vue-json-pretty :data="unserialize(flow.output)"></vue-json-pretty>
+                <vue-json-pretty v-if="hasPayload(flow.output)" :data="unserialize(flow.output)"></vue-json-pretty>
+                <div v-else class="text-muted small">
+                    This run completed without returning a value.
+                </div>
             </div>
         </div>
 
@@ -648,8 +654,12 @@
                     </div>
                 </div>
 
+                <div v-if="!timelineRows().length" class="text-muted small">
+                    No history events have been recorded for this run yet.
+                </div>
+
                 <div
-                    v-if="runDetailTab() === 'timeline'"
+                    v-else-if="runDetailTab() === 'timeline'"
                     class="timeline-events timeline-events-virtual"
                     :style="{ height: historyViewportHeight + 'px' }"
                     @scroll="onHistoryScroll"
@@ -2143,6 +2153,32 @@ export default {
             return value !== null && value !== undefined && value !== ''
         },
 
+        hasPayload(raw) {
+            if (raw === null || raw === undefined || raw === '') {
+                return false
+            }
+
+            const decoded = this.unserialize(raw)
+
+            if (decoded === null || decoded === undefined) {
+                return false
+            }
+
+            if (typeof decoded === 'string') {
+                return decoded !== ''
+            }
+
+            if (Array.isArray(decoded)) {
+                return decoded.length > 0
+            }
+
+            if (typeof decoded === 'object') {
+                return Object.keys(decoded).length > 0
+            }
+
+            return true
+        },
+
         hasObjectEntries(value) {
             return value !== null
                 && value !== undefined