fleet-manager ui: target version as a dropdown (fleet-known + custom)

Replace the free-text target-version field with a select populated from versions
known across the fleet (each host's running + targeted version, newest first),
plus a 'custom...' escape to a text input for a version not yet seen. Avoids
typos on the pin. (Authoritative serving-side source is a follow-up.)

serving/fleet-manager/ui/src/pages/Lifecycle.svelte

@@ -11,8 +11,30 @@
   let busy = $state(false)
   let confirmOpen = $state(false)
   let rebootMode = $state('none') // none | now | at
+  let customVersion = $state(false)
   let form = $state({ target_version: '', kiosk_url: '', reboot_at: '', drain: false })
 
+  // Versions known across the fleet (running + already-targeted), newest first.
+  // A "custom…" escape covers a version not yet seen anywhere. (Future: source
+  // from the serving-side release inventory / deployment manifest.)
+  let versions = $derived.by(() => {
+    const s = new Set()
+    for (const h of hosts) {
+      if (h.version?.image_version) s.add(h.version.image_version)
+      if (h.desired?.target_version) s.add(h.desired.target_version)
+    }
+    return [...s].sort((a, b) => b.localeCompare(a, undefined, { numeric: true }))
+  })
+  function onVersionSelect(e) {
+    const v = e.target.value
+    if (v === '__custom__') {
+      customVersion = true
+      form.target_version = ''
+    } else {
+      form.target_version = v
+    }
+  }
+
   async function load() {
     try {
       const d = await api.get('/api/hosts')
@@ -26,6 +48,7 @@
     selected = h
     okMsg = ''
     rebootMode = 'none'
+    customVersion = false
     form = {
       target_version: h.desired?.target_version || '',
       kiosk_url: h.desired?.kiosk_url || '',
@@ -90,7 +113,18 @@
         <div class="grid grid-cols-1 gap-3 sm:grid-cols-2">
           <label class="block">
             <span class="tag">target version</span>
-            <input class="mt-1 w-full rounded border border-line bg-void2 px-2 py-1.5 text-sm text-ink" bind:value={form.target_version} placeholder="e.g. 0.5.0" />
+            {#if customVersion || !versions.length}
+              <div class="mt-1 flex items-center gap-2">
+                <input class="min-w-0 flex-1 rounded border border-line bg-void2 px-2 py-1.5 text-sm text-ink" bind:value={form.target_version} placeholder="e.g. 0.5.0" />
+                {#if versions.length}<button type="button" class="btn" onclick={() => (customVersion = false)}>list</button>{/if}
+              </div>
+            {:else}
+              <select class="mt-1 w-full rounded border border-line bg-void2 px-2 py-1.5 text-sm text-ink" value={form.target_version} onchange={onVersionSelect}>
+                <option value="">— unset —</option>
+                {#each versions as v}<option value={v}>{v}</option>{/each}
+                <option value="__custom__">custom…</option>
+              </select>
+            {/if}
           </label>
           <label class="block">
             <span class="tag">kiosk url</span>