feat(server): config file editor — read/edit/save a host config file per instance

The Server page's config-honesty note now leads somewhere real: a
Configuration file panel that loads a config file from the instance
(prefilled with the game's primaryConfigFile hint — server.cfg,
ServerSettings.ini, GameXishu.json), edits it in a mono textarea, and
saves it straight to the host through the jailed agent file bridge.
Not-found is handled gracefully (empty editor to create). Works across
games; gameProfiles gains primaryConfigFile per game.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/views/admin/ServerView.vue

@@ -357,6 +357,51 @@ async function refreshInstanceStatus() {
   }
 }
 
+// ---- Config file editor (reads/writes via the jailed agent file manager) ----
+const cfgPath = ref('')
+const cfgContent = ref('')
+const cfgLoaded = ref(false)
+const cfgLoading = ref(false)
+const cfgSaving = ref(false)
+const cfgError = ref<string | null>(null)
+
+// A reasonable default config-file hint per game (operator can change it).
+const cfgHint = computed(() => profile.value.primaryConfigFile ?? '')
+
+async function loadConfigFile() {
+  const path = (cfgPath.value || cfgHint.value).trim()
+  if (!path || !currentInstance.value) return
+  cfgPath.value = path
+  cfgLoading.value = true
+  cfgError.value = null
+  try {
+    cfgContent.value = await instancesStore.readFile(path)
+    cfgLoaded.value = true
+  } catch (e) {
+    // Not-found is fine — present an empty editor to create it.
+    cfgContent.value = ''
+    cfgLoaded.value = true
+    cfgError.value = e instanceof Error ? e.message : 'File not found — saving will create it'
+  } finally {
+    cfgLoading.value = false
+  }
+}
+
+async function saveConfigFile() {
+  const path = cfgPath.value.trim()
+  if (!path || !currentInstance.value) return
+  cfgSaving.value = true
+  try {
+    await instancesStore.writeFile(path, cfgContent.value)
+    cfgError.value = null
+    toast.success(`Saved ${path}`)
+  } catch (e) {
+    toast.error(e instanceof Error ? e.message : 'Failed to save file')
+  } finally {
+    cfgSaving.value = false
+  }
+}
+
 async function toggleAutomation(field: 'crash_recovery_enabled' | 'auto_update_on_force_wipe' | 'force_wipe_eligible') {
   if (!server.config) return
   const newValue = !server.config[field]
@@ -1040,6 +1085,44 @@ onMounted(async () => {
       </form>
     </Panel>
 
+    <!-- Config file editor — reads/writes via the jailed agent file manager -->
+    <Panel v-if="currentInstance" title="Configuration file" subtitle="Edit a config file directly on the host (jailed to the instance)">
+      <div class="sv__cfg-row sv__mb-sm">
+        <Input
+          v-model="cfgPath"
+          :placeholder="cfgHint || 'path/relative/to/instance/root'"
+          class="sv__cfg-path"
+          :mono="true"
+        />
+        <Button
+          variant="secondary"
+          icon="folder-open"
+          :loading="cfgLoading"
+          :disabled="(!cfgPath && !cfgHint) || cfgLoading"
+          @click="loadConfigFile"
+        >Load</Button>
+        <Button
+          v-if="cfgLoaded"
+          icon="check"
+          :loading="cfgSaving"
+          :disabled="!cfgPath || cfgSaving"
+          @click="saveConfigFile"
+        >Save</Button>
+      </div>
+      <Alert v-if="cfgError" tone="info" class="sv__mb-sm">{{ cfgError }}</Alert>
+      <textarea
+        v-if="cfgLoaded"
+        v-model="cfgContent"
+        class="sv__cfg-editor"
+        spellcheck="false"
+        rows="16"
+      ></textarea>
+      <p v-else class="sv__cfg-hint">
+        Load <code>{{ cfgHint || 'a config file' }}</code> to view and edit it. Changes are written
+        straight to the host through the agent — jailed to this instance's directory.
+      </p>
+    </Panel>
+
     <!-- Automation -->
     <Panel title="Automation">
       <div class="sv__toggles">
@@ -1127,6 +1210,23 @@ onMounted(async () => {
 .sv__controls { display: flex; flex-wrap: wrap; gap: 10px; }
 .sv__mt-sm { margin-top: 12px; }
 .sv__instance-pick { display: flex; align-items: center; gap: 12px; }
+.sv__cfg-row { display: flex; gap: 10px; align-items: center; }
+.sv__cfg-path { flex: 1; }
+.sv__cfg-editor {
+  width: 100%;
+  min-height: 320px;
+  background: var(--surface-base);
+  color: var(--text-primary);
+  border: none;
+  box-shadow: var(--ring-subtle);
+  border-radius: var(--radius-md);
+  padding: 14px 16px;
+  font-family: var(--font-mono);
+  font-size: var(--text-xs);
+  line-height: 1.6;
+  resize: vertical;
+}
+.sv__cfg-hint { margin: 0; font-size: var(--text-sm); color: var(--text-tertiary); line-height: 1.55; }
 .sv__select {
   background: var(--surface-base);
   color: var(--text-primary);