feat(logs): date filter + clear log endpoints

- API GET /api/logs now accepts from_ts / to_ts (unix epoch, half-open
  [from, to)) so callers can scope by arbitrary time range.
- API DELETE /api/logs added. Same from_ts / to_ts semantics. No params
  = wipe everything and reset the AUTOINCREMENT counter.
- Dashboard Logs page: date picker that scopes both the view and the
  delete button to the selected day in the user's local timezone. The
  clear button is red and confirms before deleting; label switches
  between "전체 로그 초기화" and "<날짜> 하루치 삭제".

frontend/src/api.js

@@ -32,5 +32,11 @@ export const api = {
     ).toString()
     return req('/logs' + (q ? `?${q}` : ''))
   },
+  clearLogs: (params = {}) => {
+    const q = new URLSearchParams(
+      Object.fromEntries(Object.entries(params).filter(([, v]) => v !== '' && v != null))
+    ).toString()
+    return req('/logs' + (q ? `?${q}` : ''), { method: 'DELETE' })
+  },
   restart: () => req('/proxy/restart', { method: 'POST' }),
 }