fix(web-ui): remove placeholder option from day detail select

Remove the "选择日期" placeholder from the day detail dropdown and
auto-select the first available day when usage data loads.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: SurviveM

web-ui/modules/app.methods.session-browser.mjs

@@ -849,6 +849,9 @@ export function createSessionBrowserMethods(options = {}) {
                 if (loadSucceeded) {
                     this.sessionsUsageLoadedOnce = true;
                     this.sessionsUsageLoadedLimit = limit;
+                    if (!this.sessionsUsageSelectedDayKey && Array.isArray(this.sessionUsageDailyTableRows) && this.sessionUsageDailyTableRows.length > 0) {
+                        this.sessionsUsageSelectedDayKey = this.sessionUsageDailyTableRows[0].key;
+                    }
                 }
             }
         },

web-ui/partials/index/panel-usage.html

@@ -96,9 +96,8 @@
                                     <div class="usage-daydetail-controls">
                                         <select
                                             class="usage-daydetail-select"
-                                            :value="sessionsUsageSelectedDayKey || ''"
+                                            :value="sessionsUsageSelectedDayKey || (sessionUsageDailyTableRows[0] && sessionUsageDailyTableRows[0].key) || ''"
                                             @change="selectSessionsUsageDay(($event.target && $event.target.value) ? String($event.target.value) : '')">
-                                            <option value="">{{ t('usage.dayDetail.pick') }}</option>
                                             <option v-for="day in sessionUsageDailyTableRows" :key="'day-select-' + day.key" :value="day.key">{{ day.key }}</option>
                                         </select>
                                         <button